Experimental Analysis of Performance of Heat Exchanger with Plate Fins and Parallel Flow of Working Fluids

Transcription

1 World Journal of Engineering and Tehnology, 2017, 5, ISSN Online: ISSN Print: Experimental Analysis of Performane of Heat Exhanger with Plate Fins and Parallel Flow of Working Fluids Drilon Meha *, Arben Avdiu, Fejzullah Krasniqi, Ali Muriqi, Xhevat Berisha Faulty of Mehanial Engineering, University Hasan Prishtina, Pristina, Kosovo How to ite this paper: Meha, D., Avdiu, A., Krasniqi, F., Muriqi, A. and Berisha, X. (2017) Experimental Analysis of Performane of Heat Exhanger with Plate Fins and Parallel Flow of Working Fluids. World Journal of Engineering and Tehnology, 5, Reeived: May 31, 2017 Aepted: July 17, 2017 Published: July 20, 2017 Copyright 2017 by authors and Sientifi Researh Publishing In. This work is liensed under the Creative Commons Attribution International Liense (CC BY 4.0). Open Aess Abstrat Heat exhangers are devies in whih heat is transferred from one fluid to another fluid as a result of temperature differene. Heat exhanger presented in the urrent paper in whih inside the tubes flows water, but outside the tubes flows air aims to enable ooling of irulating water, whih serves to ool the engine of a mahine. Suh exhangers find appliation in the automotive industry as well as heating and ooling equipment and HVAC systems et. The surfae of the heat exhanger by the air side always tends to be muh larger using surfae fins in order to failitate equalization of thermal resistane for both sides of the heat exhanger, beause the rate of transmission of heat from the water side is muh greater. Furthermore, the paper will present analytial and experimental studies involved for determination of performane of plate-fin heat exhanger for various flows of working fluids in order to get the highest values of performanes i.e.: overall heat transfer oeffiient U, effiieny of heat exhanger ɛ, maximal and real heat transferred, pressure drop, air veloity and Reynolds number from the air side of heat exhanger et. The present sientifi paper is based on the fat that from the experimental model made for laboratory onditions, onlusions are derived that an be used during installation of suh heat exhanger on ertain mahines in order to predit their performane. Keywords Heat Exhanger, Heat Transfer, Fins Surfaes, Single Phase, Plate Fins, Performane 1. Introdution Heat exhanger air-water, applied to the urrent paper, due to the flow of fluids in the same diretion is alled the heat exhanger with the parallel flow [1]. DOI: /wjet July 20, 2017

2 Regarding the working fluids mixtures, the same heat exhanger is alled with mixers along exhanger from the air side, and without mixture from the water side [1]. In suh heat exhangers fins surfaes from the air side, whih have found more appliations are tubular and retangular ones [1] [2]. The overall effiieny of heat exhangers with fins surfaes is influened by many fators suh as the surfae material of heat exhangers, fluid flows, plaing distane of surfaes fins, surfaes number of fins and fluid types, flow diretion et. Fins surfaes are usually plaed outside the tubes, but there are some appliations when they are plaed inside the tubes [1]. Most of the researh papers in the analysis of plate fins heat exhangers are based on pressure drop and heat transfer harateristis. A heat exhanger with plate fins surfaes onsists of plates from the air side instead of tubes to separate the hot and old water. In 1930 s plate heat exhangers are used to meet the hygieni demands of the food industry. These days plate fins heat exhangers find appliations in wide range of fields as power generation, heating, ventilation and air onditioning systems, treatment of waste heat, gas prodution, hemial industry, pharmaeutials, food industry et. A method whih provides an ideal platform for studying the performane of plate fins heat exhanger with misible and immisible systems was developed by M. Thirumarimurugan [3]. An experimental investigation for laboratory onditions is developed by Alur [4] in order to test heat transfer and pressure drop harateristis for a plate fin heat exhanger with the ounter flow. Nabadi [5] has analyzed a numerial investigation of pressure drop and heat transfer in a heat exhanger that was designed with the different shape of pin fins. The purpose of this paper is to determine the optimal operation of the platefin heat exhanger for the various flow of working fluids in order to ahieve the highest values of performane. The experimental set up in this investigation onsists of a parallel flow heat exhanger. Changing the airflow along the tunnel is done by a variable speed axial fan, the values of whih are measured by a differential miromanometer. At the exit of the tunnel, the air is warmed by reeiving heat from the water flowing inside the tubes in the same diretion. Water flows are provided by a irulating pump, where the amount of water in the system is measured by a rotameter plaed at the exit of hot water. The heat exhanger s effetiveness is alulated for different values of the flow rate between working fluids. The temperature measurements are read on eletrial ontrol panel display. Fins surfaes analyzed in the urrent paper, for heat exhanger air-water are retangular. Therefore, the results of the heat transfer and the pressure drop harateristis in funtion of hanging the flow of working fluids are presented below. 2. Thermal Analysis of Heat Exhangers with Parallel Flow The effiieny of the heat exhanger with parallel flow of working fluids is alulated by the expression: 1 e ε = NTU ( 1+ R) ( 1+ R) (1) 436

3 The overall heat transfer oeffiient, determined experimentally, is derived from the basi equation of heat transferred: Q exp Uexp = (2) A LTMD Q exp, [W] experimental heat transferred; A, [m 2 ] overall surfae of heat transfer; Logarithmi mean temperature differene for parallel flow of working fluids is alulated by the expression: LTMD = ( Thi, Ti, ) ( Tho, To, ) ( Thi, Ti, ) ln ( Tho, To, ) T h, in, [ C] hot fluid temperature at the entrane of heat exhanger; T, [ C] hot fluid temperature at the exit of heat exhanger; ho, T, in, [ C] old fluid temperature at the entrane of heat exhanger; T o,, [ C] old fluid temperature at the exit of heat exhanger; The ratio of the thermal apaity of working fluids is given by the expression: m R = (4) m h, [kj/kg K] speifi heat apaity of old fluid: h, [kj/kg K] speifi heat apaity of hot fluid: m h, [kg/s] flow mass of hot fluid; m, [kg/s] flow mass of old fluid; The number of transmission units: Uexp A NTU = (5) C U exp, [W/m 2 K] the overall heat transfer oeffiient determined experimentally; Cmin = min ( Ch, C), [kj/kg K] minimal thermal apaity of fluid; The maximum temperature differene in a heat exhanger: T = T T (6) h min max h, in, in The maximum heat transferred in a Heat exhanger is: Q = C T (7) max min max Aordingly, the effiieny of heat exhanger water-air with plate fins surfaes is: Q exp ε = (8) Q The total heat transferred between working fluids with parallel flows in a heat exhanger with plate fins surfaes, is alulated by the following equation: Q = ε m t (9) exp max From the expression of the number of transmission units, we an extrat the max (3) 437

4 value of the produt U * A: Uexp A = min m min NTU (10) 3. Pressure Drop by the Air Side of Heat Exhangers with Plate Fins Surfaes The maximum pressure drop is onsidered as one of main the design speifiations. If the pressure drop reahes the maximum values higher than allowed, additional fins surfae should not be added. For the desription of pressure drop is neessary to apply for non-dimensional numbers: Staton, Prandtl, and Reynolds: α p µ G Dh St =, Pr =, Re = (11) G λ µ p µ, [Pa s] dynami visosity of fluid; D h, [m] hydrauli diameter; λ, [W/mK] thermal ondutivity of fluid; G, [kg/m 2 s] the mass veloity or mass flux; α,[w/m 2 K] heat transfer oeffiient with onvetion; p, [kj/kg K] speifi heat apaity of the fluid The hydrauli diameter is defined as four times the flow passage volume divided by the total heat transfer area: 4 A L Amin Dh = = 4 (12) P A P, [m] perimeter of setion; A min, [m 2 ] minimum flow area. The pressure drop from the air side in heat exhangers with Plate fins surfaes is given by expression [2]: 2 G 2 ρ b A ρb 2 ρ b p = ( k + 1 σ ) f ( 1 ke σ ) (13) 2 ρb ρ j Amin ρ ρj k, k e oeffiient of pressure losses in the entrane and exit of the heat exhanger [1]. m, [kg/s] flow mass of air; ρ, [kg/m 3 ] The average density of air; ρ b, ρ j, [kg/m 3 ] fluid density in the entrane and exit of heat exhanger; f The oeffiient of frition. The oeffiient of frition from the air side of plate fin heat exhanger (f) is alulated by the expression: ( ) 0.2 f = Re (14) Where: σ = Amin A, the minimum surfae free flow/frontal area A 4 L =, (total area of heat transfer/minimum flow area) Amin Dh L, [m] distane of flow in heat exhanger; L A min, [m 3 ] minimal volume of free flow; 438

5 A, [m 2 ] the overall surfae of heat transfer. The mass veloity or mass flux is defined as: ρ w A ρ w G = = (15) A σ min w, [m/s] average veloity of air, The average density of the air: ρ = + 2 ρb ρ j (16) 4. Testing Unit of Water/Air Heat Exhanger Installed in a Sheet Steel Tunnel This model unit presented in Figure 1 makes possible to study the operation of water/air heat exhanger, made of aluminum with round expanded pipes, installed in a painted sheet steel tunnel. The irulation of air is ensured by a variable speed fan, Four Pt100 heat resistors, onneted to a digital instrument, are plaed at suitable measuring points in the system. The air flows through the test unit 1 (tunnel) by means of the axial fan 2, and a differential miromanometer 3, interloked with a alibrated flange, makes it possible to measure the rate of flow from the fins plate surfaes. On the other hand, the water with temperature T 1 enters into Heat Exhanger and leaves it with temperature T 2. Further, water is sent to the water feed tank 4, where the water level measurement is performed by float type valve 5. By means of the three speed irulating pump 7 the water is sent to the eletri heater 10 in whih the water of temperature is inreased to T 1. Water flow measurement is arried out by a flow meter 11. Figure Testing tunnel with alibrated diagram and water/air heat exhanger, 2. Eletrially operated variable speed axial fan, 3. Differential miromanometer, 4. Water irulation and feed tank, 5. Float type valve, 6. Tank disharge valve, 7. Three-speed irulation pump, 8. Safety valve for boiler, 9. Boiler disharge valve, 10. Eletri boiler, 11. Flowmeter (0 to 300) [l/h], 12. Flow ontrol valve, 13. Air bleed valve, 14. Eletrial ontrol panel, LI. Level indiator, T 1. Water temperature at the inlet to the heat exhanger, T 2. Water temperature at the outlet of the heat exhanger, T 3. Air temperature at the inlet to the heat exhanger, T 4. Air temperature at the outlet of the heat exhanger. 439

6 The valves 12, 13, 18, 19, 6 serve to maintain the water iruit under the permissible norms in order to perform a normal operation of the heat exhanger. 5. Experimental Analysis of Performane of Plate-Fin Heat Exhanger Experimental analysis of performane for the urrent heat exhanger is made in the devie shown in Figure 1. In order to highlight the impats of hanging flow of working fluids, the following diagrams are presented bellow as: logarithmi mean temperature differene, effiieny of heat exhanger, real and maximal heat transferred in heat exhanger, the overall heat transfer oeffiient, pressure drop, air veloity, Reynolds number from the air side of heat exhanger. Although, the physial properties of the working fluids, whih are presented in the Table 1 with a fixed values, during the alulation they are taken into onsideration being hanged with temperature, even though the effet of temperature on physial properties has little or there was no effet on the performane of heat exhanger. As seen from Figure 2(a), the inrease in the flow of hot water from 30 to 40 [l/h] auses a derease in LMTD, while the inrease in flow from 40 to 200 [l/h] auses a slower growth of LMTD. The maximum hange of LMTD for variable flow mass of hot water (for unhanged air flow 40 [kg/h]) has resulted to be equal with C C = 3.23 C. Similarly, in Figure 2(b). LMTD is displayed with the hanging of air flow, whih flows through the heat exhanger from 40 to 220 [kg/h], but for unhanged hot water flow 100 [l/h]. From fig. 3b.are seen the flutuations of LMTD as a result of flow mass of air hanging. This hange has aused the maximum differene of LMTD from C C = 2 C. From omparisons between Figure 2(a) and Figure 2(b), we ame to the onlusion that the effet of hanging the water flow (3.23 C) is more pronouned than the hange of air flow (2 C) to the LMTD. From Figure 3(a), it is seen that for unhanged hot water flow 100 [l/h] when the air flows through the heat exhanger with fins surfaes is inreased by the fan, in that ase, the overall effiieny of the heat exhanger is redued. This is due to the short ontat between the working fluids and the inability that air mass flow to absorb that heat. The same happens during the drive of a ar when the veloity is too large in that ase, it auses the engine to warm up, thus preventing the ooling. Therefore, for suh appliations, the Heat Exhangers should be used with phase hanges of working fluids. The opposite ours in Figure 3(b), when the airflow generated by the fan remains unhanged, while hanges the flow of hot water through the three-speed irulation pump. Beause of the air flows slowly, then Table 1. Physial properties of working fluids along the Plate-fin heat exhanger. k k Cp Cp μ μ ρ ρ Pr Pr W/mK W/mK J/kgK J/kgK Pa * s Pa * s kg/m 3 kg/m Water Air Water Air Water Air Water Air Water Air

7 Figure 2. Graphial presentation of logarithmi mean temperature differene (LMTD method), by hanging the mass flow of working fluids in a heat exhanger. Figure 3. Graphial presentation of the effiieny of the heat exhanger (plate fins heat exhanger) determined experimentally by hanging the flow mass of working fluids. it reeives more heat and onsequently the effiieny of Heat exhanger inreases. Figure 4(a) shows the atual and maximal heat transmitted, measured experimentally in laboratory onditions, with variable air flow and onstant flow ofhot water. In Figure 4(a) with the red line is presented the heat that air reeives by hanging the flow mass of air from 293 [W] up to 1188 [W], while with blue line is presented the maximum heat that air an absorb. Similarly, another diagram Figure 4(b) was onstruted when the hot water flow was hanged (m air = onst), but in this ase, the heat absorbed by air flow was too low and reahed the values from 59 [W] to 81 [W]. From Figure 5(a), it an be seen that the inrease in the air flow by the air side with fins surfaes has a muh more signifiant effet on the overall heat transfer oeffiient than the hange in the flow of hot water, whih after 50 [l/h] has almost onstant value (Figure 5(b)). Figure 6 shows the pressure drop by the fins surfaes applying the Equation (13). The highest experimental value of the pressure drop has resulted in 1.5 [bar] and for these high values of airflow undesirable noise was produed. With the hange of mass flow of air with the fan, the air veloity on the side with fins surfaes has been linearly hanged (see Figure 7). Based on the geometri dimensions of the Heat exhanger and the working fluid properties, the determination of air veloity from 0.32 [m/s] to 1.72 [m/s] has been made possible. With the inrease of air veloity is inreased the oeffiient of heat onvetion from the air side. Figure 8 presents the Reynolds number from the fins surfaes by hanging the air flow. As an be seen from Figure 8, the hange of air flow in the value of Reynolds number is linear. As muh higher the Reynolds number to be the value of heat exhanged between working fluids will be greater. 441

8 Figure 4. Real and maximal heat transferred in a heat exhanger, by hanging the flow mass of working fluids. Figure 5. The overall heat transfer oeffiient determined experimentally depending on the mass flow of working fluids. 442

9 Figure 6. Pressure drop from the air side of the heat exhanger by hanging the mass flow of air. Figure 7. Air veloity from the air side of a heat exhanger by hanging the mass flow of air. Figure 8. Reynolds number from the air side of a heat exhanger by hanging the mass flow of air. 6. Conlusions One of the key parameters in inreasing the plate fins heat exhanger performane is to hange the mass flow of working fluids. Figure 3 shows the effet of hanging the flow of working fluids to the overall heat transfer effiieny. From this figure, it has been shown that for unhanged flow mass of hot water, but for a variable mass flow of air by the side of a heat exhanger with fins surfaes, the 443

10 effiieny of the heat exhanger is redued. This has happened as a result of short ontat between working fluids and the inability that air to absorb the heat from the mass flow of hot water. The result of inreased airflow is the impossibility of ahieving desired ooling (a ase presented in the automotive industry that has aused the rising of engine temperature). The opposite happens when m air = ons, but hanges the mass flow of hot water by ausing the heat exhanger effiieny to inrease. Furthermore, from (Figure 4) it is onluded that the atual and maximal heat transferred in a heat exhanger for m air ons and m h = ons, have values muh higher than the ase when m h onst and m air = onst. This means that the greatest effet of the heat transferred in heat exhanger air-water is the hange of air flow from the fins surfaes. In addition, the overall effiieny of heat transfer oeffiient determined experimentally varies onsiderably by hanging the air flow from the side with fins surfaes and a small hange is observed if the mass flow of hot water is hanged (Figure 5). Referenes [1] Shah, R.K. and Sekuli, D.P. (2003) Fundamentals of Heat Exhanger Design. Wiley. [2] Kakaç, S. and Liu, H. (2002) Heat Exhangers, Seletion, Rating and Thermal Design. CRC Press, Boa Raton. [3] Tharumarimurugan, M. and Kannadasan, T. (2009) Simulation Studies on Plate Type Heat Exhanger Using ANN. Internaional Journal of Chemial Teh Researh, [4] Alur, S. (2012) Experimental Studies on Plate Fin Heat Exhangers. Dotoral Dissertation, National Institute of Tehnology, India. [5] Nabadi, H. (2008) Optimal Pin Fin Heat Exhanger Surfae. Dotoral Dissertation, Shool of Sustainable Development of Soiety and Tehnology. Submit or reommend next manusript to SCIRP and we will provide best servie for you: Aepting pre-submission inquiries through , Faebook, LinkedIn, Twitter, et. A wide seletion of journals (inlusive of 9 subjets, more than 200 journals) Providing 24-hour high-quality servie User-friendly online submission system Fair and swift peer-review system Effiient typesetting and proofreading proedure Display of the result of downloads and visits, as well as the number of ited artiles Maximum dissemination of your researh work Submit your manusript at: Or ontat wjet@sirp.org 444