A Study Of Thermal Performance Of Heat Pipe Heat Exchanger In Building Heat Recovery

Interest in the use of heat pipe technology for heat recovery and energy saving in a vastrange of engineering applications has been on the rise in recent years.Heat pipes are considered to be super conductors,due to this feature it allows them to be able to operate under nearly isothermal conditions and increases the thermal performance of heat exchangers.Different types of wick structure have been studied in different applications,but the sintered capillary wick has not been widely studied in the building application.The objective of this study is to focus on the thermal performance of a special type of wick structure which is the sintered powder heat pipes heat exchanger in building applications with varying working conditions and to optimize the effectiveness with various design variables.For the purpose of heat recovery this study uses a heat pipe with a total length of 1.2[m]having the evaporator and condenser length of 0.5[m]and the adiabatic length of 0.2[m].Rectangular fins with a length of 0.29[m]in staggered arrangement is used.Four rows of heat pipes were installed in the normal arrangement to the flow.Five major types of heat transfer limitations of heat pipe are introduced to validate the model presented in this study.These limitations are capillary limit,Entrainment limit,viscous limit,sonic limit and the boiling limit.The heat pipe heat exchanger is designed according to the thermal network approach.This approach is an engineering tool that has reliable accuracy.It is combined with the effectiveness-NTU method to predict the thermal performance of heat pipes.This numerical simulation is performed in EES.Optimization study is carried out on the basis of Genetic algorithm method that is available in EES.Mutation rate,Number of population,and the number of individuals are the operators of genetic algorithm and are set to control the optimization process.The objective function is set as the effectiveness of heat pipe and is maximized in this study.Three Decision variables(design)which are outside diameter,length Ratio(L_e/L_c),number of rows are selected to search the global optimum point before terminating the process.The result of simulations are as given:The heat pipe modelled for this study is wellunder all the heat transfer capacity limits.The Internal thermal resistance for condensation and evaporation are calculated 0.039 and 0.099[K/W]for summer and 0.029 and 0.089 for winter[K/W],these are the dominant resistances affecting the performance.Wick parameters such as permeability and thermal conductivity have been varied to analyze the performance.Wick permeability increased the heat transfer capacity of heat pipes,whereas the effective thermal conductivity decreased the heat transfer capacity.The summer and winter values of effectiveness decrease with the increase of capacity ratio.The lowest effectiveness occurs at capacity ratio closest to 1.Increasing NTU(number of transfer units)from 0.1-0.5 gives a linear increase in the effectiveness.Effectiveness increases with NTU in the ratio of 9:10.The inlet temperature is increased in the range of 298-313[K]keeping the outlet cold side temperature constant for both winter and summer conditions.Both summer and winter effectiveness increases with the increase in temperature,following the trend that winter values have the highest effectiveness.The optimization results from Genetic algorithm show that the optimal effectiveness is achieved at 0.7279.This value is the mean optimal value of heat pipe throughout the year.The findings from this study reveals that effectiveness is at the lowest when the Capacityratio approaches zero.Wick permeability and thermal conductivity of wick are contradicting parameters and designer has to make tradeoff between these parameters according to the desired application.The length of evaporator should be kept slightly higher than the condenser length in order to achieve optimal effectiveness.This proposed study will provide guidelines for design of sintered capillary wick heat pipes in building heat recovery.