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Title: | Studies on Heat Transfer Characteristics of Heat Pipe Heat Exchanger Natural Convection |

Authors: | Kumar, Vikas |

Supervisor: | Gangacharyulu, D. Tathgir R.G. |

Keywords: | Heat Pipe Heat Exchanger;Tilt Angle;Heat Transfer;Analytical Model;Natural Convection;Mechanical Engineering |

Issue Date: | 14-Sep-2006 |

Abstract: | An analytical model has been developed to evaluate the thermal performance of a heat pipe heat exchanger (HPHE) under natural convection by adopting thermal resistance approach. The model evaluates the rate of heat transport and pressure drop across evaporator system of the HPHE under natural convection. The model computes various thermal resistances of the heat pipe at the external surface of evaporator & condenser as well as at the internal surface of the heat pipe based on the correlations available in the literature. The rate of heat transport has been calculated by converting the model into a computer programme, whose solution is based on an iterative procedure. A test rig has been developed for thermal performance evaluation of a single heat pipe under natural convective conditions as well as to do the validation of the analytical model for the same. Natural convective experimental studies have been carried out on the heat pipe at different tilt angles from the horizontal, i.e., 15 o , 20 o , 25 o , 30 o , 35 o , & 90 o and various temperatures of the heating fluid, i.e., 40 o C, 50 o C, 60 o C, & 70 o C at a constant flow rate of the heating fluid (laminar flow regime) in the evaporator section at an ambient temperature of about 1 1 o C. The experimental observations reveal that the tilt angle and the heating fluid temperature influence the heat transport rate of the heat pipe. The heat transport rate increases as the tilt angle of the heat pipe increases from 15 o to 25 o , and it starts decreasing beyond 25 o . The maximum heat transport rate of the heat pipe has been obtained at a tilt angle of 25 o and 70.3 o C heating fluid temperature, which may be due to the lowest overall thermal resistance at this operating condition, which is caused by the minimal external condenser thermal resistance of the finned condenser. Some typical experimental results on different individual heat transfer coefficients have been compared with the analytical model for a single heat pipe at 25 o tilt angle. The evaporator heat transfer coefficient predicted by using Dobson & KrÃ¶gerÃ s correlation has closer agreement with the experimental value as compared to FandÃ s correlation, therefore, the computed evaporator surface temperature is very close (within 1 o C) to the experimental value. The internal heat transfer coefficient of the heat pipe predicted by the correlation provided by Chi, is 3.5 to 4.7 times more as compared to the experimental value, therefore the predicted condenser temperature is on the higher side by 3.1 o C to 5.6 o C as compared to the experimental value. The heat transfer coefficient on the condenser side predicted by the correlation proposed by Churchill and Chu (for all range of Rayleigh number) is close to the experimental value. Even though, there is a large deviation in the prediction of internal heat transfer coefficient of the heat pipe, the overall heat transfer coefficient predicted by the analytical model is in good agreement with the experimental value. In fact, the value of internal heat transfer coefficient is very high and it contributes little to the overall heat transfer coefficient, which is mostly governed by condenser heat transfer coefficient and to a less degree by evaporator heat transfer coefficient. The analytical model validated on a single heat pipe has been extended to the HPHE by incorporating appropriate geometrical & heat transfer correlations. Another test rig has been developed for evaluating the thermal performance of a HPHE under natural convective cooling condition as well as for validating the analytical model. The experiments have been conducted on the HPHE under natural convective condition at different tilt angles from the horizontal (15 o , 20 o , 25 o , 30 o , & 90 o ) and at various heating fluid temperatures (40 o C, 50 o C, 60 o C, & 70 o C) at its evaporator inlet. The Reynolds number of heating fluid in the evaporator section varied in the range of 436 to 1380. The variation of ambient surrounding temperature was in the range of 18 o C to 22 o C. The experimental studies show that the heat transport rate of the HPHE increases marginally as the Reynolds number of heating fluid increases in the evaporator section because the condenser heat transfer coefficient does not increase significantly. The maximum heat transport rate from the HPHE has been obtained at 25 o tilt angle and 70 o C heating fluid temperature. Some typical results of heat transport rate obtained from experiments for the HPHE at 25 o tilt angle have been compared with the results predicted by the analytical model. It has been observed that the heat transport rate predicted by the analytical model is 7 to 27 more than he heat transport rate of the HPHE obtained from the experiments. The excess heat transport rate prediction is due to computation of higher value of internal heat transfer coefficient by the analytical model and the model predicts in turn higher condenser temperature. The pressure drop across tube bundle in the evaporator section of the HPHE is small in magnitude and majority of the pressure drop is due to sudden expansion & contraction of the heating fluid at the inlet & outlet of the evaporator section. The deviation in the computed pressure drop and the experimental value is within the range of 2 to 13 . The present investigation concludes that the developed analytical model for thermal performance evaluation of the HPHE is reasonably in good agreement with the experimental results. It is expected that the analytical model along with computer programme can be used as a design tool for thermal performance analysis and rating of the HPHE under various configurations & operating conditions for natural convective applications. |

Description: | Ph.D. Thesis |

URI: | http://hdl.handle.net/123456789/62 |

Appears in Collections: | Doctoral Theses@MED |

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