RESEARCHES OF HEAT EXCHANGE OF A WIRE-AND-TUBE CONDENSER FOR THE INREASE OF REFRIGERATOR S EFFICIENCY

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2 KAUNAS UNIVERSITY OF TECHNOLOGY LITHUANIAN ENERGY INSTITUTE IGNAS HOFMANAS RESEARCHES OF HEAT EXCHANGE OF A WIRE-AND-TUBE CONDENSER FOR THE INREASE OF REFRIGERATOR S EFFICIENCY Summary of Doctoral Dissertation Technological Sciences, Energetics and Power Engineering (06T) 2014, Kaunas

3 This doctoral dissertation was prepared in at Kaunas University of Technology, Faculty of Mechanical Engineering and Mechatronics, Department of Thermal and Nuclear Energy. Scientific supervisor: Prof. Dr. Vytautas DAGILIS (Kaunas University of Technology, Technological Sciences, Energetics and Power Engineering, 06T). Board of Energetics and Power Engineering Science field: Prof. Dr. Habil. Stasys ŠINKŪNAS (Kaunas University of Technology, Technological Sciences, Energetics and Power Engineering, 06T) Chairman; Assoc. Prof. Dr. Habil. Algirdas KALIATKA (Lithuanian Energy Institute, Technological Sciences, Energetics and Power Engineering 06T); Prof. Dr. Habil. Gvidonas LABECKAS (Aleksandras Stulginskis University, Technological Sciences, Energetics and Power Engineering 06T); Assoc. Prof. Dr. Stasys SLAVINSKAS (Aleksandras Stulginskis University, Technological Sciences, Transport Engineering 03T); Dr. Egidijus URBONAVIČIUS (Lithuanian Energy Institute, Technological Sciences, Energetics and Power Engineering 06T). Official opponents: Prof. Dr. Habil. Vytautas MARTINAITIS (Vilnius Gediminas Technical University, Technological Sciences, Energetics and Power Engineering 06T); Prof. Dr. Habil. Gintautas MILIAUSKAS (Kaunas University of Technology, Technological Sciences, Energetics and Power Engineering 06T). The official defense of the dissertation will be held at 2 p.m. on 12 December, 2014 at the public meeting of the Board of Energetics and Power Engineering Science field in the Dissertation Defense Hall at the Central building of Kaunas University of Technology. Address: K. Donelaičio St , LT Kaunas, Lithuania. Phone (370) , fax. (370) , doktorantura@ktu.lt Summary of dissertation sent out on 12 November, Dissertation is available at the libraries of Kaunas University of Technology (K. Donelaičio str. 20, Kaunas) and Lithuanian Energy Institute (Breslaujos str.3, Kaunas). 2

4 KAUNO TECHNOLOGIJOS UNIVERSITETAS LIETUVOS ENERGETIKOS INSTITUTAS IGNAS HOFMANAS VIRBAIS BRIAUNUOTO KONDENSATORIAUS ŠILUMOS MAINŲ TYRIMAI ŠALDYTUVO EFEKTYVUMUI PADIDINTI Daktaro disertacijos santrauka Technologijos mokslai, energetika ir termoinžinerija (06T) 2014, Kaunas 3

5 Disertacija rengta m. Kauno technologijos universitete, Mechanikos ir mechatronikos fakultete, Šilumos ir atomo energetikos katedroje. Mokslinis vadovas: Prof. dr. Vytautas DAGILIS (Kauno technologijos universitetas, technologijos mokslai, energetika ir termoinžinerija 06T). Energetikos ir termoinžinerijos mokslo krypties taryba: Prof. habil. dr. Stasys ŠINKŪNAS (Kauno technologijos universitetas, technologijos mokslai, energetika ir termoinžinerija 06T) pirmininkas; Doc. habil. dr. Algirdas KALIATKA (Lietuvos energetikos institutas, technologijos mokslai, energetika ir termoinžinerija 06T); Prof. habil. dr. Gvidonas LABECKAS (Aleksandro Stulginskio universitetas, technologijos mokslai, energetika ir termoinžinerija 06T); Doc. dr. Stasys SLAVINSKAS (Aleksandro Stulginskio universitetas, technologijos mokslai, transporto inžinerija 03T); Dr. Egidijus URBONAVIČIUS (Lietuvos energetikos institutas, technologijos mokslai, energetika ir termoinžinerija 06T). Oficialieji oponentai: Prof. habil. dr. Vytautas MARTINAITIS (Vilniaus Gedimino technikos universitetas, technologijos mokslai, energetika ir termoinžinerija 06T); Prof. habil. dr. Gintautas MILIAUSKAS (Kauno technologijos universitetas, technologijos mokslai, energetika ir termoinžinerija 06T). Disertacija bus ginama viešame energetikos ir termoinžinerijos mokslo krypties tarybos posėdyje 2014 m. gruodžio 12 d. 14 val. Kauno technologijos universiteto centrinių rūmų disertacijų gynimo salėje. Adresas: K. Donelaičio g , LT Kaunas, Lietuva. Tel. (370) , faksas (370) , el. paštas Disertacijos santrauka išsiųsta 2014 m. lapkričio 12 d. Su disertacija galima susipažinti Kauno technologijos universiteto (K. Donelaičio g. 20, Kaunas) ir Lietuvos energetikos instituto (Breslaujos g. 3, Kaunas) bibliotekose. 4

6 INTRODUCTION The climate warming and the depletion of the ozone layer are the problems that have been causing particular concern during the last three decades since the Montreal Protocol was signed. After signing the Montreal Protocol, a lot of attention was directed at the refrigeration technology operating by vapour compression cycle, which is often referred to as compressor refrigeration technology. Such refrigeration technology is special in terms of ecology, because its negative effect on the environment has even three aspects. One of these negative effects is caused by refrigerant circulating in refrigeration systems, which may deplete the ozone layer [1] Research has shown that refrigerants which are more friendly in this respect are less efficient and therefore, new possibilities to increase the efficiency of refrigerators had to be found. Eventually, all refrigerants containing chlorine molecules will be prohibited. Namely chlorine molecules cause chain reactions in the outermost layers of the atmosphere, where a single molecule can destroy several thousand ozone molecules [2]. However, two other aspects of negative environmental effects remained: they cause the climate warming. Refrigeration technology induces the climate warming in two ways. One of them is related to the material of the refrigerant itself. Beside the negative effect on the ozone layer, some refrigerants may cause the greenhouse effect that is hundreds and thousands of times greater than the effects of CO 2 gas, the emissions of which are strictly regulated by the relatively recent (2005) Kyoto Protocol. The third aspect of the significant negative environmental effect is that the compressor refrigeration technology is operating by the reverse thermodynamic cycle, which consumes energy. Thus, the effect of the vapour compression cycle technology on the environment is related to relatively high energy needs. The production of electric energy is directly related to CO 2 emissions and other polluting chemical compounds. Reduction of electric energy consumption is encouraged not only by stricter environmental requirements, but also the depleting fossil fuel resources and more expensive energy. The above causes induce research in the field of saving, ecology, and increasing the efficiency. In our geographical area, up to 30 % of electric energy consumed domestically is used by domestic refrigeration appliances. Meanwhile in the countries, where domestic air conditioners are inevitably used, this proportion is even higher. EU directives of the latter decades have demanded to increase the efficiency of these domestic appliances, i.e. to reduce electric energy consumption, more and more strictly. In 1994, the EU Directive 94/2/EEC came into effect, which introduced the efficiency classes and severely restricted the sales of domestic appliances, if their efficiently class was not compliant with the requirements of the Directive. Later on, in 2002, this Directive was replaced by yet a stricter one EEC/02/99, 5

7 which increased the requirements for efficiency. Two additional energy classes were introduced: A+ and A++. We must admit that due to the adoption of the directives, electric energy consumption of domestic refrigerators decreased by half per one liter of useful capacity in less than ten years. For example, in 1997, Snaigė RF310 refrigerator used to consume 1.44 kwh of electric energy per 24 hours. By 2005 electric energy consumption of this refrigerator had decreased to 0.67 kwh per 24 hours. Electric energy consumption requirements for domestic refrigeration appliances are getting stricter. At the moment, there is a new Directive in effect, which has introduced the requirements for A+++ energy class. It is likely that the requirements will be even stricter. Therefore, the manufacturers of domestic refrigerators are forced to constantly decrease the electric energy consumption of these appliances. Now is the time to look for methods to achieve A+, A++, and A+++ energy classes of refrigerators at the lowest possible cost (due to market competition). One of the methods is to increase the efficiency of one of the heat exchangers condensers of the refrigeration system. The condenser transfers the heat absorbed by the refrigerant from the refrigeration area through the vaporizer to the surroundings. The efficiency of its heat transfer determines the operation of the compressor and in turn, the electric energy consumed by it. The decrease of the temperature of the condenser causes the decrease of the condensation pressure of the refrigerant, which leads to shorter operation time of the compressor and lower electric energy consumption. In modern domestic refrigerators most often wire-and-tube condensers are used. A wire-and-tube condensers is a tube bent into a serpentine shape with crossing metal wires. The issues of increasing the efficiency of heat transfer of such condensers are analysed in the present dissertation. The possibilities to achieve A+, A++, and A+++ energy classes of the refrigerators are also presented. Object of research heat exchange of wire-and-tube condenser of domestic refrigerator with its limiting environment. The aim of the work to identify the conditions of more efficient heat transfer of wire-and-tube condenser and to evaluate their influence on the efficiency and reliability of a domestic refrigerator. Tasks to achieve aim of research: 1. To perform theoretical and experimental researches in order to determine the efficiency of condenser heat exchange under conditions of natural convection. 2. To determine the heat transfer coefficient under mixed convection and define its impact on the variation of condensation temperature. 6

8 3. To evaluate the impact of reduced condensation temperature on the efficiency of domestic refrigerator efficiency. 4. To substantiate the technical potentiality to achieve A+, A++, A+++ refrigerator energy classes with minimal additional costs of production on the basis of experimental researches. 5. To create the visualizations of wire-and-tube condenser and distribution of its limiting temperature fields by applying,,ansys CFX program. 6. To evaluate the impact of reduced condensation temperature on the longevity of regenerative heat exchanger. Scientific novelty of work The field of science of refrigeration equipment was expanded by the results of experimental research substantiating more efficient transfer of condensation heat by mixed convection from the surface of a wire-and-tube condenser. Relevance of the work Efficient energy consumption is one of the main tasks of energy modernization in order to implement the objective of long-term sustainable energetic. The rapidly growing need of appliances working by inverse thermodynamic cycle (refrigerators, air conditioners, heat pumps) increases the energy consumption that directly influence the rate of release to environment of climate warming CO 2 gases. Household refrigerators consume nearly one-third of household electricity, thus these devices deserve special attention it is aimed to optimize the energy efficiency of household refrigerators. One of the methods to do that is to create the more efficient heat exchange between condenser and environment air without changing the basic production technology of this appliance. Modern production technology of wire-and-tube condenser is very efficient and that results to its low prime costs. It would be irrational to refuse the appliance corresponding to such technical characteristics in production process. Therefore it is necessary to find a solution of more efficient use of existing design condenser by creating more intense air movement influencing more efficient heat Exchange between the external surface of the condenser and environment air. Modern condenser is not only the efficient technologies, but also the heat exchanger of optimal geometric dimensions. The change of any external conditions requires detailed and consistent research, as well as careful modelling in order to get the maximum higher heat transfer with the lowest metal consumption. 7

9 Practical value of the work Installation of a low power ventilator in the refrigerator system allows obtaining more intense heat transfer in mixed convection, which has a significant effect up to 8 % for A+++ and up to 10 % for A++ energy class refrigerators and allow achieving these energy classes. Refrigerator one manufacturer will introduce this technology in their production. Affirmations for defense 1. The integral heat transfer coefficient of the whole condenser found by researches allows determining the heat transfer coefficients of its wires and tubes. The research also enabled to define the impact of its limiting environment to heat exchange. 2. In case of mixed convection and when compared to conditions of natural convection, the impact of condenser limiting environment is reduced. When evaluating the cost of metal of the condenser to heat flow unit, the mixed convection is suitable for condenser of existing design. 3. The usage of low capacity fan determines the more efficient heat transfer from condenser of researched type while maintaining its unchanged design and technology, as well as without damaging the balance and stability of refrigerator system. 4. More effective transfer of condensation heat in mixed convection leads to decrease of electricity consumption and their cost of production of refrigerators of various energy classes (A+, A++, A+++) and makes the appliance more attractive to the buyer. 5. The graphical analysis of patterns of heat exchange variations in case of mixed convection and efficiency coefficient of wire and tube can be easily created by the visualizations of external surface of wire-and-tube condenser distribution of its limiting temperature fields prepared by using general purpose fluid dynamics package Ansys CFX. 6. In case of lower condensation temperature the temperature of heat exchanger heat recovery ventilation lowers to dangerous limit of corrosion cause. Lower condensation temperature of cooling agent does not cause the corrosion of heat recovery ventilation if the heat is supplied from the cover of compressor. 8

10 Approbation of the scientific work Five publications on the theme of doctoral dissertation were published: one of them published in science journal included into ISI Web of Science with citation index. Two of them are in Lithuanian Science Council approved list of international databases publications. Four publications belong to list of republics and international conference proceedings. Author s contribution in analyzed problem The author I. Hofmanas and the research advisor V. Dagilis have designed, produced, calibrated and started three experimental stands. One stand investigated the heat transfer of wire-and-tube condenser and the value of heat transfer coefficient was established, when water was used as a heat transfer medium. The other stand investigated the influence of the surroundings of the condenser on its heat transfer and the value of heat transfer was also established, when the condensation process takes place in the condenser and its surface is similar to isothermal. The third stand was used to identify the effect of heat transfer of a mixed convection condenser on the operation and electric energy consumption of a refrigerator. Scope of the scientific work 5 chapters with sub chapters, total volume is 111 pages, 52 figures, 12 tables. 1. LITERATURE REVIEW Despite of the widespread use of wire-and-tube condensers and their decisive influence on the capacity and economy of refrigerators, very little literature on the investigations of their heat transfer has been published. It is also noted by the majority of the authors of published works. In most works, the heat transfer of wire-and-tube condenser under natural convection conditions is investigated. Probably the greatest contribution to the research of wire-and-tube condenser heat transfer has been made by Tanda and Tagliafico (1997) [1]. The authors of this work [1] carried out the experiments under the conditions of natural convection. Based on the experiment results, they derived a semiempirical formula for the calculation of the Nusselt number, which evaluated the geometric parameters of the wire-and-tube condenser: the diameter and spacing of the wires and the diameter and spacing of the tubes. The majority of later works refer to the methodology of heat transfer calculation presented in this work [1]. To author s knowledge, openly available literature does not contain any works, which would present the investigations of heat transfer of wire-and-tube condenser under the conditions of mixed convection or which would evaluate the 9

11 influence of such heat transfer on the efficiency of the refrigerator. There are several works published [2, 3], which present the investigations of heat transfer of wire-and-tube condenser under the conditions of forced convection. Different from the present work, the speed of forced air in the latter works is up to 10 times higher and the component of natural convection is completely disregarded. 2. EXPERIMENTAL EQUIPMENT AND RESEARCH METHODOLOGY To achieve the aim of the work a series of experimental investigations had to be carried out. Two experimental stands were designed and produced to carry out the experimental investigations. Another two experimental stands in the common laboratory of KTU and Snaigė were used after adjusting them to fit the purpose of the investigations. Having established that the mixed convection yielded the expected results, a construction that allows implementing the conditions of mixed convection in the environment of a refrigerator condenser was designed. Thus, yet another experimental stand was created to determine the influence of mixed convection on the efficiency of a refrigerator Research methodology for condenser heat transfer under the conditions of natural convection The methodology for investigating the condenser heat transfer using a special stand To determine the value of the heat transfer coefficient of wire-and-tube condenser an experimental stand was designed and produced. Its principal scheme is presented in Figure 1. 10

12 Fig. 1. The scheme of condenser experimental investigations: 1 water tank; 2 electric heater; 3 contact thermometer; 4 water discharge control valves; 5 wire-and-tube condenser; 6 mercury thermometers; 7 discharge measurement vessel; 8 scales Water temperature of 40 ºC is maintained in the tank 1 by the electric heater 2, which is controlled by the contact thermometer 3. The valves 4 maintain constant water level to ensure even water discharge through the condenser. Water temperatures before and after the condenser are measured by mercury thermometers with the accuracy of one tenth of a degree. Water discharge was measured with sufficiently high accuracy by collecting the water to the vessel 7 and weighing it on the scales 8. Five experiments were carried out The methodology for investigating the influence of condenser heat transfer and the surroundings on its heat transfer The experiments were carried out in the common laboratory of KTU and Snaigė with a domestic refrigerator RF-31. The condenser is fitted to the back side of the refrigerator within a distance of 0.03 m from it (Fig. 2). 11

13 Wire Refrigerator back-wall Condenser tube Room wall Air current Fig. 2. The air flow is being limited by the wall and refrigerator back wall s The experiments were carried out in a special thermal camera, where the standard temperature of 25 C was kept during all tests. The walls of the thermal camera are insulated and it was presumed that the temperature of the walls is the same as that of the air, i.e. 25ºC; therefore, the influence of the walls was not evaluated. The refrigerator was moved close to the camera wall with the distance of s=0.05 m to the back wall of the refrigerator. Temperature of the surface of the condenser was measured at three different places upper part, middle and lower part, t cd1, t cd2, t cd3 respectively. The temperatures are registered by a computer every 15 seconds and their values are automatically marked in the dependence of time and temperature. The t cd2 temperature corresponds the condensing temperature the best, because temperature t cd1 may be of somewhat higher in the upper part of the condenser and t cd3 of 1 to 3 C lower the lower part. At the upper part the condensing process is not started yet and is finished at the lower part. When measuring the temperatures of the surface of the condenser (t cd1, t cd2, t cd3 ) and changing the distance s, it was investigated how the surroundings of the condenser influence its heat transfer. Later on, the experiments were continued with a condenser placed in an unrestricted space, where the condenser is bent by an angle of 90º from the back wall of the refrigerator (Fig. 3). 12

14 ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ t c1 ~ ~ t a1 t a2 t c2 t a3 t a4 t a5 t c3 0,3 m Fig. 3. View of experimental refrigerator with the bent condenser avoiding heat influence from compressor shell In order to experimentally determine the average heat transfer coefficient of the condenser, it is necessary to find heat capacity Q. The coefficient is determined as a ratio of the Q to temperature difference multiplied by area fcd, i.e. Q /( f cd t). The total heat exchange area is known: f cd = m2. Temperature difference between the condenser and the ambient air was measured at several locations, and the average difference Δt was determined. Measurements of ambient air temperature were done at five different heights from the room floor starting 0.3 m for the first height. The temperature sensors were placed in five equal distances at each 0.3 m. The three termocouple sensors were carefully tested. The second sensor t cd2 corresponds the condensing temperature the best. The heat capacity of condenser was calculated as follow cd cd2 cd1 Q g h h (2.1) 13

15 As we can see, the heat capacity depends on mass flow capacity of refrigerant R600a (isobutene) g cd. The enthalpies h cd1 and h cd2 are ingoing and outgoing refrigerant enthalpies. The enthalpies were estimated when temperatures t cd1, t cd2, t cd3 were measured during the test. The temperature was measured in each 15 seconds. The amount of circulating refrigerant, i.e. mass flow g cd was determined during special calorimetric test of the compressor. The developed testing rig in the meant united KTU and Snaigė laboratory allows carry out calorimetric tests (Fig. 4) at various conditions. The description of the stand is presented in [4]. Moreover, it was possible to fix measurements during the test but not only at the end of the calorimetric process. So in this case the capacity and efficiency is being determined at refrigerator s working conditions but not at standard conditions (CECOMAF or ASHRAE). These conditions are sufficient different. For example, the compressors must be tested at standard conditions when temperature of condensation is 55 C, while refrigerator s condensing temperature does not exceed 40 C. Therefore, when the 14 Fig. 4. General view of the compressor calorimeter stand condensation temperature is 36.7 C and evaporating temperature C (it corresponds evaporating temperature at refrigerator cycle conditions minus 25 C) the determined HTK80AA compressor cooling capacity Q o = 109 W and effectiveness Evaporating temperature C was estimated during calorimetric test of the refrigerator when hydraulic losses of the suction line were evaluated. During refrigerator test conditions when evaporating temperature exceeds -25 C precisely, the pressure inside the compressor shell is estimated by the special pressure-gauge. According the CoolPack the evaporating pressure at -25 C is bar. The difference between bar and measured pressure is due hydraulic losses in the trunk-line between evaporator and compressor. So

16 estimated pressure value of bar corresponds the boiling temperature equal to minus 27.6 C. The amount of circulating refrigerant g cd is determined as g cd Qo / h 1 h 4 (2.2) where h 1, h 4 are enthalpies of vapor and liquid at the compressor testing condition, which corresponds refrigerator working conditions, i.e. gas suction temperature t 1 = 22 C and liquid temperature t 4 = t cd3 = 34.8 C The methodology of investigation of mixed convection on the efficiency of the refrigerator Having ascertained by tentative experimental investigations that the mixed convection yields the expected results, we have moved on to the design and production of the construction that would allow implementing such external heat transfer of a condenser in a domestic refrigerator. When designing this construction, many factors had to be taken into account, such as: reliability, fitting, simplicity of production, inexpensiveness, and many other technical aspects and production processes of a refrigerator. Having evaluated the above mentioned and not mentioned aspects in the common laboratory of Snaigė and KTU, the ventilator fitting construction (bracket) was produced and installed in a refrigerator. Fig. 5 illustrates the construction. Ventilator bracket Fig. 5. General view of the back wall of the refrigerator Having implemented the conditions of mixed convection in a domestic refrigerator RF34, the influence of this heat transfer on the efficiency of the 15

17 refrigerator was experimentally determined. The experimental investigation stand in the same laboratory was used; this stand is compliant with the parameters and possibilities of such stands of many European refrigerator producers. The experimental stand is universal enough and did not need to be specifically modernized for these experiments. The temperatures were measured by thermocouples, the accuracy of which is ±0.2 C on the average, but they may be additionally calibrated for each experiment using the accurate mercury thermometers with the gradation of 0.01 C. The stand allows measuring and registering 24 temperatures at different locations or surfaces of the refrigerator. The temperatures are registered in the computer by the software developed specifically for the stand. It allows not only graphical depiction and drawing of temperature curves, but also the control of the refrigerator according to the selected temperature. In such case, the selected temperature must be very stable, i.e. not to respond to various electric or magnetic disturbances, which are most numerous when the refrigerator is switching on. Therefore, special protection agaunst the above disturbances was created in the laboratory. Moreover, thermocouple beads were additionally insulated by special material, which is very resistant to mechanical and temperature effects. The radiation effect of the thermocouples was avoided by enwinding the contact place by opaque tape and when measuring the temperatures in open space, the thermocouples were placed in a brass cylinder, the blacklevel (coefficient) of which did not exceed 0.1. The plugs of the thermocouples ensured very good and reliable contact in their sockets. The experimental chamber of the refrigerator was insulated both thermally and from noise. The ambient (i.e. experimental chamber) temperature was also maintained using the above software. It allows not only maintaining the set temperature in the chamber, but also the relative humidity, controlling the air cooler, and maintaining low ambient temperature, if necessary, e.g. +16 C or +10 C. The software of the stand has yet another improvement in comparison with the standard experimental stand. The programme was improved to register the temperature at the set frequency. Therefore, the temperature may be registered at exceptionally high frequency, when it is necessary to record the transitional modes of refrigerator operation. In this case, the smallest time interval of temperature registration and refrigerator control was 0.2 seconds. 16

18 3. RESULTS OF EXPERIMENTAL INVESTIGATION 3.1. Research of heat exchange of rod condenser in case of natural convection Using the experimental stand Fig.1, when water acted as the heat transfer agent, the obtained results are provided in Table 1. Table 1. Results of condenser s heat exchange Input temperature t v1, o C Output temperature t v2, o C Ambient temperature t o, o C Logarithm Δt, o C Water debut g v, g/min Heat efficiency Q, W. Coefficient of heat release, W/(m 2 K) The average heat release coefficient, obtained experimentally, is lesser than the coefficient calculated using the semi-empirical equation of Nu figure, provided in the paper [1]. In this case, the experimental results differ from the theoretically calculated results approximately by 18 %. The authors of the paper [1] carried out the experimental studies at a much larger difference of the temperature between the water entering the condenser and the ambient air. During the recent experiments of the authors [1], this difference was up to 46 C. In this case, during the experiment a small difference of temperature was maintained, such as it usually occurs during the operation of the refrigerator (15 C). This is one of the main reasons of the obtained mismatch. Later experiments were carried out using the refrigerator RF-31 (Fig. 2, Fig. 3). In this case the process of condensation takes place in the condenser and its surface is close to isothermal. The experiment allows investigating the effect of a limiting space for a heat exchange. In Fig. 6 the dynamics of a temperature during the couple of working cycles is shown. It can be seen that mean condensing temperature lowered by almost 2 C when distance between room wall and condenser was enlarged up to 1 m. The Fig. 7 demonstrates the condensing temperature change in case when the condenser is bent from compressor shell. The position enables to avoid the influence of rising heat from compressor. The temperature t cd3 decreased much because of subcooling effect. Heat transfer at the lower part of the condenser is of somewhat different in this case. Rising cool air gets out heat from refrigerant liquid but not from gas. The heat transfer of such kind is not 17

19 isothermal one as it is while refrigerant condensates. As it can be seen the mean temperature t cd2 (as well as t cd1 ) is lower by approximately 2 C compare to previous and by almost 4 C compared to conventional position of the condenser. It means that the heat from compressor has sufficient influence on condensation temperature t cd2. Fig. 6. Condensing temperature with conventional air space (first working cycle) and when it was enlarged (second two cycles) Fig. 7. The change of condensing temperatures when the condenser is bent to avoid heat from compressor shell (as it shown in Fig. 3.) In the case of a natural convection conventional space, the condensing temperature was fixed 40.3 C. When the condenser was bent from compressor (see Fig. 3.) the condensing temperature was 36.7 C. The first case is not strictly defined due to the impact of the compressor: operation temperature of the compressor is high and slightly increases the temperature in surrounding of the condenser. Therefore, experiments were carried out with a non limiting space, when the condenser was placed like is shown in Fig. 3. Then the temperature of evaporating dropped to -25 C, the measured temperatures correspond values as follows: t cd1 = 37.3 C, t cd2 = 36.7 C, t cd3 = 34.8 C. Accordance to CoolPack software, the gas and liquid enthalpies are h cd1 = kj/kg, h cd2 = kj/kg respectively. The temperatures around the condenser were slightly different from an average temperature of thermo camera, which appeared due to the air stratification. The average air temperature during the test was equal to 24.1 C and temperature difference in respect of condensing temperature t cd2 Δt = 12.6 C. 18

20 Gas suction temperature (22 C) was measured during the standard test of the refrigerator. Within these conditions we have enthalpies: h 1 = kj/kg, h 4 = kj/kg and amount of circulating refrigerant g cd = kg/s. Then heat capacity of the condenser (according Eq. (2.1)) is Q = W. According to the equation Q /( f cd t), the coefficient of heat transfer is =11.1. W/(m 2 K). This quantity is slightly smaller than that theoretically estimated (see previous author s article, Ref. [1]), which was =12.6 W/(m 2 K) Results of numerical heat transfer investigation The simulation was accomplished by the Ansys CFX software. First of all it was determined temperature distribution on the surfaces of wire-tube condenser that allow analyze fin (wire) effectiveness visually. Air heat transfer coefficient makes an impact on the fin effectiveness [1] and at the same time the difference of the temperatures between the wire and air, i.e.: tanh( mpt / 2 ) 4 E and m. (3.1) mpt / 2 wd w The difference of temperatures between the outer surface of condenser and air around it may decrease or increase local heat transfer coefficient α as well as average one. Modeling the above mentioned software allowed us to get the thermovision view of the surface of condenser (Fig. 11). The temperature of wires is lower compare to tube temperature because of fin efficiency lower than unity. Fins or wires (in our case) form over 2/3 total area of condenser. Temperature decreases across the edge of the wire from 38.8 o C to 35.9 o C. Meanwhile the tube temperature varies in much narrow interval: from 38.8 o C at a contact with tube, to 39.2 o C where the wires are not attached. In the middle of a bend the temperatures increases up to 39.7 o C, while condensing temperature inside is 40.0 C. The average temperature of the condenser is calculated as well. At the done conditions the average surface temperature is 37.4 o C. 19

21 Temperature o C Temperature o C Fig. 8. View of temperature distribution of the condenser It is quite difficult to study a space of the heat transfer of a condenser experimentally. The space contains a high dynamics of air temperature and velocity. In other words, each point of space has a different air velocity, temperature and other parameters. Air velocity measurement is difficult, also because it has a low value the highest average velocity at the top of the condenser, under the conditions of natural convection, amounts to 0.6 m/s. And it is even more difficult to measure air velocity between the condenser tubes and wires. Therefore, numerical investigations of this space and heat transfer of the condenser were carried out in natural and mixed convection. Using Ansys CFX program, the visualizations of the distribution of temperature fields and velocities were obtained (Fig. 9-12). When designing the model, some problems were encountered. It was impossible to simulate 20

22 the heat transfer of an entire condenser (of actual size) due to immoderate number of finite elements. Therefore, 1/5 part (20 %) of the condenser height and width was simulated. When designing the model, the boundary conditions were adopted. The condenser is located between two non-slip and two absolutely slippery surfaces. Thus, part of the condenser element in the model is like in a rectangular channel, which corresponds to the condenser space. In the beginning of calculations, the air temperature in the condenser space was 25 o C. Two walls which are parallel to the plane of the condenser correspond to the properties of the refrigerator wall and the room wall. The distance between these walls is 0,05 m, and the condenser is in the middle of this distance. The other two walls were used to limit the condenser space and, at the same time, to simplify the calculations. The temperature of the condenser tube was 40 o C, and the entire surface of the condenser was isothermal these conditions correspond to the actual operating conditions of the condenser. Although a small part of the condenser was simulated, by using the visualizations obtained, it is possible to observe the patterns of the condenser heat transfer. The Fig. 9 presents air temperature spread around vertical surface of the condenser. The figure demonstrates the thermovisional image of numerical simulation results. As we can see, the highest temperature in the rise direction of the flow is behind the tubes. The results (visualizations) of numerical experiments showed that increasing the condenser height increases the areas of the highest air temperature behind the tubes. The temperature of the overall space surrounding the condenser also increases. Higher air temperature reduces heat transfer coefficient from external side of the condenser. The heat transfer coefficient α depends on temperature difference between the surfaces and fluid because Ra=Gr Pr number determines the α in natural convection case. The expression of the Rayleigh number (Ra) is equation: 2 a c a a 3 Ra g( Tcd Ta ) H. (3.2) a a c The calculation at natural convection heat transfer requires thermal and physical fluid properties that form Gr but not Re numbers. However, for the local heat transfer calculation the calculation of local velocity is necessary. The used program allows to calculate the local air velocity, although fluid moves at a very low velocity by natural convection. Thermovisional image of numerical simulation air velocities is presented in Fig. 10. As can be seen the horizontal tubes make slower velocity of air. For example, the velocity at cross-flow direction (very close the tubes from the upper side) is only about 0,05 m/s. On the other hand the cross-flow area is relatively small in comparison with total area of the condenser. And these parts have negligible influence on the heat transfer. 21

23 Fig. 9. The image of the distribution of temperatures of the air surrounding the condenser element under the conditions of natural convection Fig. 10. The image of the distribution of velocity of the air surrounding the condenser element under the conditions of natural convection As it was mentioned above, it is difficult to measure air velocity, therefore a of numerical experiments with Ansys CFX program were carried out. The results obtained, i.e. visualizations, allowed carrying out the analysis. Fig. 11, 12 presents several results when the air was given the initial velocity of movement of 0.1 m/s. As we can see in Fig. 12, air velocity with regard to Y axis is symmetrical, i.e. uniformly distributed on both sides of the tubes in vertical direction. This is influenced by artificially created air movement; due to the influence of the walls, the air velocity increase closer to the tubes is noticed in y direction. As the air velocity near the walls decreases and the boundary layer increases, the air velocity increase closer to the tubes is noticed in y direction. This is due to the heat transfer. Obviously, the increase of air velocity is due to heat exchange (Fig. 10, 12). 22

24 Fig. 11. The image of the distribution of temperatures of the air surrounding the condenser element under the conditions of mixed convection, when the air was given the initial velocity of movement of 0,1 m/s Fig. 12. The image of the distribution of velocity of the air surrounding the condenser element under the conditions of mixed convection, when the air was given the initial velocity of movement of 0,1 m/s Air temperature distribution (Fig. 11) with regard to y axis is also symmetrical. In this case, the average temperature between the wires and tubes is only about 1.2 o C lower than in the case of natural convection. However, more than 2/3 of the condenser space in the z direction is filled with air, the temperature of which is 25 o C. Therefore, the average temperature of the condenser space under the conditions of mixed convection is close to 25 o C, while the temperature of the condenser space under the conditions of natural convection is higher by 3 o C. In the case of mixed convection, the average velocity between the tubes and wires is about % higher than corresponding velocity of natural convection. All of the foregoing leads to more efficient heat transfer compared to that under normal conditions. In this case, the average convection heat transfer coefficient is about 60 % higher, than that of natural convection, when the initial air velocity is 0.1 m/s Results of the investigation of mixed convection effect Created ventilator mounting structure (model) (Fig. 5) may also be attributed to the results of the work. This ventilator mounting structure allowed 23

25 Temperature, o C implementing the heat transfer of mixed convection in the environment of refrigerator condenser and experimentally investigating the impact of the heat transfer on refrigerator efficiency. First, the impact of different power ventilators on the surface temperature of the condenser was tested. The results of this test are presented in Fig. 13. Ventilator power, W Higher than the compressor s background noise Time Fig. 13. The change of condensation temperatures depending on the ventilator power in the interval of one operation cycle Fig. 13 indicates that the highest impact is made by the lowest power ventilator. The ventilator with the power of just 0.68 W creates the conditions of mixed convection and thus, decreases the condensation temperature from 35.5 o C to 32 o C. Therefore, the consuption of electric energy decreases by 5.1 % after the evaluation of the electric energy consumed by the ventilator. After increasing the ventilator power twice, the effect is weaker: just one degree and respectively the decrease of total electric energy consumption by 6.9 %. The noise emitted by thus ventilator (1.35 W) approximates the noise of the compressor, which is not permitted due to ergonomic characetristics of the refrigerator. The overall dynamics of refrigerator temperatures in natural and mixed convection of different intensity is presented in Fig. 14 (analogous to Fig. 13.). 24

26 Temperature, o C Condensation temperature Ambient temperature Temperatures in the storage compartment Vaporizer surface temperatures Temperatures in the freezer compartment Time Fig. 14. Dynamics of characteristic refrigerator temperatures in natural and mixed convection of different intensity The above Fig. 14 indicates that only the condensation temperature changes out of all refrigerator temperatures. It means that forced air convection from the side of the condenser is minor and does not affect the distribution of the refrigerant between the vaporizer and the condenser. In other words, the boiling temperature of the refrigerant does not change due to the lower condensation temperature. With intense air movement the refrigerant would accumulate in the condenser. Lower condensation pressure slows down the flow of the refrigerant through the capillary tube, which causes its accumulation in the condenser. In such case, there is a lack of refrigerant in the vaporizer. The vaporizer operates with lower efficiency, because its transfer area is not completely exploited. IT is a phenomenon generally called the violation of the temperature balance. In case of the lack of refrigerant, there would be a lack in the vaporizer of the storage chamber. This would cause an increase of air temperature of this chamber, whereas the required temperature in low-temperature chamber would not change. The above statements are confirmed by other tests, the aim of which was to determine the stability of refrigerator operation mode in a longer operation interval. Fig. 15 indicates that the change of the ventilator operation mode immediately changes the condensation temperature, which then remains constant, if a stable air temperature of the chamber is maintained. From the obtained dependence (Fig. 15) it is obvious that the temperatures in the storage (positive) and low-temperature (negative) chambers remains stable. It means that the circulation of the refrigerant hardly changes, i.e. the refrigerant does not accumulate in one of the heat exchangers the vaporizer or the condenser. 25

27 Refrigerator energy consumption, kwh/p Šaldytuvo el. suvrtojimas, kwh/p Temperature, o C Otherwise, stable temperatures of the refrigerator chambers would not be obtained. Fig. 15. The change of all characteristic temperatures depending on the intensity of mixed convection in the interval of long-term testing Fig. 16 presents the results of experimental measurements, where under the conditions defined by the standard the consumption of electric energy of an A+++ energy class refrigerator depending on the intensity of mixed convection was measured. A+++ energy class Ventiliatoriaus galia, W Ventilator power, W Time Fig. 16. Electric energy consumption of A+++ energy class refrigerator RF34 depending on the ventilator power refrigerator is an exclusive or top class refrigerator with very low consumption of electric energy. It is achieved not only by the efficient refrigeration system but also by very efficient thermal insulation. The so-called superinsulation is used in the refrigerator, i.e. vacuum panels with the thermal conductivity coefficient that is several times lower than the conductivity of regular insulation. It allows reducing the heat influx to the refrigerator cabinet by almost one third. Therefore, a low-volume

28 The percentage decrease of electricity, % Elektros energijos sumažėjimas, % compressor was used. For instance, for a RF34 refrigerator a 5.5 cm 3 volume compressor instead of a 7 cm 3 one was used. Under refrigerator operation conditions (condensation temperature +40 C and mean boiling temperature - 25 C), the power of the 5.5 cm 3 compressor HXK55AA was W and 14.3 % lower than the refrigeration power of compressor HXK70AA used in A++ refrigerator. In this case, the power consumed by A+++ refrigerator decreased from 56.2 W to 49.3 W. Therefore, the power consumed by the ventilator is relatively higher as the compressor capacity significantly decreases. For instance, if a ventilator with the power of 1 W in an A++ class refrigerator accounts for 1.8 % of all consumed electric energy, for an A+++ class refrigerator this component increases to 2.1 %. When analysing the Ventiliatoriaus galia, Ventilator power, W Fig. 17. The percentage decrease of electricity consumption depending on the power of ventilator (the consumption of energy part of ventilator is estimated) dependence of the electric energy consumption of A+++ energy class refrigerator on the intensity of mixed convection (i.e. the power of the ventilator), we see that if the component of forced convection increases, an indirect dependence is obtained, which indicates that further increase of the ventilator power would hardly have any effect. It is somewhat paradoxical knowing that the heat transfer coefficient of forced convection is directly proportional to airflow speed 0.8 with the exponent 0.8 ( v ). However, it must be taken into account that the measurements were taken with a condenser of specific structure, which is adjusted for operation under natural convection conditions. The farther away from such operation conditions, the less optimum the condenser structure is. The decrease of efficiency of the wires is non-linear and with a certain airflow speed its further increase becomes unreasonable. It is well illustrated by Fig. 17, which shows a relative decrease of electric energy consumption of a refrigerator depending on the power of the ventilator. We can see that when the ventilator power exceeds 1.5 W, the curve of the decrease of electric energy consumption of an A+++ energy class refrigerator approximates the saturation. It means that further increase of ventilator power would decrease the energy consumed by the compressor less than it increases because of the operation of the ventilator itself. 27

29 Electric energy decrease, % El. energ. sumažėjimas,% In terms of electric energy consumption of a refrigerator, mixed convection is most efficient for the lowest class A+ refrigerators, as in this case the power consumed by the compressor is the highest. A ventilator of 1 W power with the compressor consumed power of 75 W additionally consumes only 1.3 % of electric energy (compare with the above mentioned 1.8 % and 2.1 % for A+++ and A++ class refrigerators respectively). However, mixed convection of A+ class refrigerator is not necessary. For A++ and A+++ class refrigerators mixed convection is efficient economically and commercially. Mixed convection is 12 most necessary for A+++ class refrigerators, but it 10 A++ A++ class klasė determines the lowest increase of efficiency. 8 Moreover, this increase 6 soon becomes saturated. A+++klasė A+++ class Having increased the 4 ventilator power to 2 W, 2 we achieve the stabilisation of electric 0 0 0,5 1 1,5 2 Ventilator Ventiliatoriaus power, galia, W Fig. 18. Efficiency increase of A++ and A+++ energy class refrigerators depending on the power of ventilators inducing mixed convection energy consumption. Whereas for A++ class the ventilators of slightly higher power may be used (see Fig. 18). Fig. 18 presents the results of experimental tests using the same ventilator, only its power was changed by changing the voltage in certain steps. We can see that more powerful ventilators are efficient for A++ class refrigerator. While 1.8 W ventilator hardly has any effect on A+++ class refrigerators, the effect on A++ refrigerators is rather significant. Thus, the refrigerators of different classes would need the ventilators of different power. It is an issue of completion and it does not pose any problems in mass production. It would be the same as completing the refrigerators of different classes with different compressors or heat transfer devices, including the condensers. 28