International Journal of Mechanical Engineering and Technology (IJMET) Volume 7, Issue 3, May June 2016, pp.102 111, Article ID: IJMET_07_03_009 Available online at http://www.iaeme.com/ijmet/issues.asp?jtype=ijmet&vtype=7&itype=3 Journal Impact Factor (2016): 9.2286 (Calculated by GISI) www.jifactor.com ISSN Print: 0976-6340 and ISSN Online: 0976-6359 IAEME Publication EXPERIMENTAL AND THEORTICAL STUDY OF THE THERMAL PERFORMANCE OF HEAT PIPE HEAT EXCHANGER Abdulsalam D.Mohamed Mechanical Engineering, University of Waist Aamer M.Al-Dabagh Mechanical Engineering, University of Technology) Duaa A.Diab Mechanical Engineering, University of Waist ABSTRACT Heat pipe heat exchanger (HPHE) considers one of the most useful devices for the recovery of waste heat energy. An Experimental study has been carried out on air to air HPHE constructed of thermosyphon heat pipes with distilled water as the working fluid and a fill ratio of 75% from the evaporator length. Its model was composed of 4 rows, each row contains 12 copper tubes, each tube have ID= 9.5 mm, OD=10mm and length =950 mm and the rows of tubes were arranged in a staggered manner. Aluminum wavy plate fins of 0.1mm thickness were fixed among the tubes to increase the heat transfer area. Tests were conducted at various flow rates (air flow rate through evaporator and condenser sections) ranged between 0.12 and 0.37 kg/s and at different temperatures of air entering evaporator section (90, 100,110) to indicate discontinuity in the effectiveness when the flow rate ratio equal to one. The results show that the maximum value of effectiveness for the four heat pipe heat exchanger occurs when = 2. Theoretical model based on (ε- NTU) has been developed by using visual basic language computer program to analysis of the temperature distribution along heat pipe heat exchanger. The results from this model were compared with experimental results. A comparison between the experimental and theoretical results shows little discrepancy. Key words: Heat Pipe Heat Exchanger, Effectiveness, Mass Flow Rate Ratio, Temperature Distribution. http://www.iaeme.com/ijmet/index.asp 102 editor@iaeme.com
Experimental and Theortical Study of The Thermal Performance of Heat Pipe Heat Exchanger Cite this Article Abdulsalam D.Mohamed, Aamer M.Al-Dabagh and Duaa A.Diab, Experimental and Theortical Study of The Thermal Performance of Heat Pipe Heat Exchanger. International Journal of Mechanical Engineering and Technology, 7(3), 2016, pp. 86 101. http://www.iaeme.com/currentissue.asp?jtype=ijmet&vtype=7&itype=3 GENERAL TERMS : Effectiveness, : actual heat transfer rate (w), : maximum heat transfer rate (w), : total overall heat transfer (w/m 2.k), : total area (m 2 ), : minimum heat capacity rate (kj/kg.k), : maximum heat transfer rate (kj/kg.k), : number of transfer unit, : effectiveness of evaporator, : number of transfer unit for evaporator, : effectiveness of condenser, : number of transfer unit for condenser, : overall heat transfer coefficient of evaporator (w/m 2.k), : heat transfer area of evaporator (m 2 ), :heat capacity rate of evaporator (w/k). : Overall heat transfer coefficient of condenser (w/m 2.k), : heat transfer area of condenser (m 2 ), : heat capacity rate of condenser (w/k), : area of pipe (m 2 ), : diameter of pipe(m), : length of pipe (m), : thickness of fin (m), : number of fins per meter, : number of pipes, :pi (=3.14), : width of fin (m), : total thermal resistance of evaporator (k/w), : external thermal resistance of evaporator (k/w), : fouling thermal resistance of (k/w), : wall thermal resistance of evaporator (k/w), : internal thermal resistance of evaporator, : total thermal resistance of condenser(k/w), : external thermal resistance of evaporator (k/w), : fouling thermal resistance of condenser (k/w), : wall thermal resistance of condenser (k/w), : internal thermal resistance of condenser, : overall effectiveness, j= number of row. 1. INTRODUCTION Heat pipe is a high performance heat transfer device which is used to transfer a large amount of heat at high rate with small temperature difference. This is achieved by evaporation of working fluid in the evaporator section and condensing in the condenser section and return of the condensate. If the wick is not used in the construction of heat pipe, it is called thermosyphon, in the thermosyphon return the working fluid from the condenser to evaporator is caused by gravity [1] Heat pipe is used in several utilizations like heat exchanger working as recovery system, solar energy communion system, cooling of electronic part, thermal control of space craft, gas turbine rotor blade cooling, etc [2]. The operating Limits of heat pipe include boiling limit, sonic limit, capillary limit, drag and viscous limit [3]. Thermosyphon Heat exchanger (THE) is one of the most productive devices for waste heat recovery. A thermosyphon heat exchanger is composed of a number of pipes arrayed in rows. It s operating with condenser section with low temperature fluid stream and evaporator section with high temperature fluid stream [4]. The advantages of Thermosyphon Heat Exchanger are Capable of operating even when temperature difference between heat source and heat sink is very low, less alimentation problems owing to containing no moving parts, large rate of effectiveness, No need any external power, Less pressure drop for fluid flow, Full disconnection of cold and hot fluids, Comparative economy and Large fidelity[5]. Zare et al. (2009) [6], studied the heat performance for air to water THE according to method. They used cold water with 0.1 kg/s flow through condenser http://www.iaeme.com/ijmet/index.asp 103 editor@iaeme.com
Abdulsalam D.Mohamed, Aamer M.Al-Dabagh and Duaa A.Diab section and hot air in closed cycle was blown into evaporator section. The temperature range of 125-225 From experimental results the effectiveness and heat transfer rate of THE were affected by heat capacity ratio for high and low temperature fluid streams.when the heat capacity ratio (Ce/Cc) of high and low temperature fluid streams was higher than unity the effectiveness increased due to ability of streams to release and absorb more heat. When the heat capacity ratio was equal to unity the effectiveness was minimum because the releasing and absorption heat was less. Danielewicz et al. (2014) [7], studied the effect of mass flow rate ratio between evaporator and condenser sections on the effectiveness of air to air THE according to method by using methanol as working fluid. The tested Heat exchanger consisted of ten rows and each row contained ten thermosyphons made of carbon steel while the fins were made from aluminum. They noticed that the effectiveness and heat transfer rate increased when the mass flow rate ratio ( increased. Vikramsinh and Mail (2015) [8], studied the thermal performance of THE for a lost air heat recovery system by using nanofluid with variable source temperature. The tested THE consisted of three rows arranged in staggered form. Each thermosyphon had inner diameter of 14.90 mm and outer diameter of 15.88mm.The Experimental tests were carried out to determine the characteristics of THE by using nanofluid. They observed that the thermal performance of THE was enhanced by replacing the conventional fluid in thermosyphon with nanofluid. A model of a multi thermosyphon heat exchanger predicted the energy savings. 2. EXPERIMENTAL TEST RIG The heat pipe heat exchanger using thermosyphon was formed and constructed as shown in figure 1. This module is composed of 4 rows; each row has 12 copper tubes of 10 mm outside diameter. The rows of tubes were staggered and connected by aluminum wavy plate fins. The fin density was 470 fins per meter. A copper tube header of 22 mm nominal diameter was connected to the ends of each row of tubes. Distilled water is utilized as working fluid in thermosyphon heat exchanger. The fill ratio is chosen to be 75% from the volume of evaporator section. A duct and fan system used for testing the performance of thermosyphon heat exchanger, Table (1.1) indicate the specifications of this system Four air heaters of 3.5 Kw each are established inside the inlet duct of evaporator section at different temperatures. All heaters are connected directly to power supply except one is connected to variac (voltage transformer), in order to control the air stream temperature. Figure 1 Schematic of heat pipe heat exchanger http://www.iaeme.com/ijmet/index.asp 104 editor@iaeme.com
Experimental and Theortical Study of The Thermal Performance of Heat Pipe Heat Exchanger Table 1.1 The specifications of duct and fan system: Cond. section Evap. section Name of section Length m) Area ( Inlet duct 1.25 0.4 0.4 Type of fan Centrifugal fan (CFM) Air flow rate control type without Outlet duct 1 0.4 0.4 Without Grill Inlet duct 1.25 0.4 0.4 Centrifugal fan (CFM) without Outlet duct 1 0.4 0.4 Without Grill 4. THEORY Formulas are dependent on developed (ε NTU) method, which is more convenient to indicate the temperatures distribution along thermosyphon heat exchanger and capable of comparing theoretical prediction and experimental results. (ε-ntu) a method depends on effectiveness of heat exchanger, (ε), which may be defined as ratio of actual heat transfer, to maximum rate of heat transfer; therefore, the effectiveness may be defined by Incropera [9]: Applying conservation of energy, general exponential function for a counter flow heat exchanger is represented by incropera [6]: The ratio is a dimensionless term called number of transfer unit which is defined as: For evaporator and condenser sections of heat pipe heat exchanger, their effectiveness can be determined respectively by the following equations: Where The heat transfer areas or for evaporator and condenser associated with one row of tubes for the evaporator and condenser can be calculated from the geometry of thermosyphon and fins as shown in Fig(2). http://www.iaeme.com/ijmet/index.asp 105 editor@iaeme.com
Abdulsalam D.Mohamed, Aamer M.Al-Dabagh and Duaa A.Diab Or Where the area of the heat is pipe without fins and is the area of the fins. Figure 2 Schematic diagram of tubes and fins for one row evaporator or condenser The overall heat transfer coefficients of evaporator and condenser sections are required to calculate the NTU of the evaporator and condenser sections. Where An Effectiveness NTU relationship for the j th row can be evaluated for the evaporator and condenser. If, are given, then three equations can be written in convenient form for solution following Azad and Geoola [10]: For C c < C e ( the condenser air streams is the minimum fluid) http://www.iaeme.com/ijmet/index.asp 106 editor@iaeme.com
Experimental and Theortical Study of The Thermal Performance of Heat Pipe Heat Exchanger For Ce < Cc (the evaporator air stream is the minimum fluid) 5. RESULTS AND DISSCUSSION To describe the discontinuity in effectiveness when mass flow rate ratio is equal one the tests were conducted. 1 In condenser section the flow rate is kept constant at minimum value (0. 132) kg/s while the flow rate in evaporator section is changed gradually from minimum value to maximum value. 2 In condenser section the flow rate is kept constant at maximum value (0. 132) kg/s while the flow rate in evaporator section is changed from minimum value to maximum value. Figures (3and4) and results are interesting for the selection of flow rate in heat recovery applications. If the processed fluid has a minimum flow rate, then higher effectiveness in heat recovery system could be achieved by using largest amount of exhaust gas flow. If the processed fluid has a higher flow rate, then the higher effectiveness in heat recovering system is achieved by using smaller exhaust gas flow rate. The right hand side curves (Fig 3) are obtained where the air stream in the condenser section is kept at low flow rate and air stream in evaporator section is http://www.iaeme.com/ijmet/index.asp 107 editor@iaeme.com
Effectivness Effectivness Abdulsalam D.Mohamed, Aamer M.Al-Dabagh and Duaa A.Diab changing from low flow rate to high flow rate. The left hand side curves (Fig 4) are obtained where the air stream in the condenser section is kept at high flow rate and air stream in the evaporator section is changing from the low flow rate to high flow rate for condenser section. Discontinuity in the effectiveness when the flow rate ratio equal to one is shown clearly in (Fig5). For predicting the temperature distribution of air across evaporator and condenser sections of thermosyphon heat exchanger. A computer program written in visual basic language as employed to generate the theoretical conditions by using iterative processes. The main purpose of the program is to calculate inlet and outlet temperature for evaporator and condenser in each row of heat exchanger. The theoretical results obtained from the program were compared with experimental data. A comparison between predicted and measured temperatures in two air streams for thermosyphon heat exchanger subjected to different values of air flow rate ratio (1.31) and (0.45) at different temperatures are illustrated in Figures (6,7, 8,and9). Figures (7and9), illustrate that the difference between experimental and theoretical results for both evaporator and condenser sections increases with increase of the inlet temperature of air to evaporator section. 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.5 1 1.5 2 2.5 3 Mass flow rate ratio Figure 3 The effect of mass flow rate ratio on for a four heat pipe heat exchanger subjected to constant flow rate in the condenser section at 0.1322 kg/s and varying evaporator flow rate 0.65 0.6 0.55 0.5 0.45 0.4 0.35 0.3 T=90 T=100 T=110 T=90 T=100 T=110 0 0.2 0.4 0.6 0.8 1 1.2 Mass flow rate ratio Figure 4 The effect of mass flow rate ratio on for a four row heat pipe heat exchanger subjected to constant flow rate in the condenser section at 0.374 kg/s and varying evaporator flow rate http://www.iaeme.com/ijmet/index.asp 108 editor@iaeme.com
Temperature (C) Temperature (C) Effectivness Experimental and Theortical Study of The Thermal Performance of Heat Pipe Heat Exchanger 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 T=90 T=100 T=110 0 0.5 1 1.5 2 2.5 3 Mass flow rate ratio Figure 5 Discontinuity of the effectiveness at flow rate ratio equal one for a four row heat pipe heat exchanger 125 100 75 50 25 Tc=Exp Tc=Theo Te=Exp Tc=Theo 0 0 1 2 3 4 5 Row number,n Figure 6 Comparison of experimental and theoretical air temperature distribution for fourrow heat pipe heat exchanger at air flow rate ratio of 1.31 at inlet temperature=110 125 100 75 50 25 Tc=Exp Tc=Theo Te=Exp Te=Theo 0 0 1 2 3 4 5 Row number,n Figure 7 Comparison of experimental and theoretical air Temperature distribution for fourrow heat pipe heat exchanger at air flow rate ratio of 1.31at inlet temperature=120 C http://www.iaeme.com/ijmet/index.asp 109 editor@iaeme.com
Temperature (C) Temperature (C) Abdulsalam D.Mohamed, Aamer M.Al-Dabagh and Duaa A.Diab 125 100 75 50 25 Tc=Exp Tc=Theo Te=Exp Te=Theo 0 0 1 2 3 4 5 Row number,n Figure 8 Comparison of experimental and theoretical temperature distribution for four-row heat pipe heat exchanger at air flow rate ratio of 0.45 at inlet temperature =110 C 125 100 75 50 25 Tc=Exp Tc=Theo Te=Exp Te=Theo 0 0 1 2 3 4 5 Row number,n Figure 5.15 Comparison of experimental and theoretical temperature distribution for fourrow heat pipe heat exchanger at flow rate ratio of 0.45 at inlet temperature=120 C 6. CONCLUSION The maximum value of effectiveness for the four heat pipe heat exchanger occurs when = 2. From solving the mathematical model there is little discrepancy between theoretical and experimental results of temperature distribution of air along THE. The difference between theoretical and experimental results for evaporator section increases with increasing evaporator air inlet temperature. REFERNCES [1] Mohamed.S, Hossien.N, and Hassan.S, Application of Heat Pipe Heat Exchanger in Heating, Ventilation and Air Conditioning System, University of Minufiya, 6,PP 1900-1908(2011). [2] Peterson. G.P, an Introduction of Heat Pipes, Modeling, Testing and Applications, New York, p.245 (1994) [3] Faghri. A, Heat Pipe Science and Technology, Taylor and Francis (1995). http://www.iaeme.com/ijmet/index.asp 110 editor@iaeme.com
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