Heat transfer characteristics of heat pipe heat exchangers for low and medium temperature heat recovery

Abstract

Heat pipe heat exchangers (HPHEs) offer many advantages over other types of heat exchangers for the recovery of heat from industrial gaseous waste streams in the low and medium temperature ranges. Heat pipes employed in industrial heat exchangers can be made with or without wicks. Although the thermal performances of single heat pipes and HPHEs have been extensively investigated, comparative studies concerning behaviours of HPHE with different types of heat pipes, at the same operating conditions, appear to be very limited. Such studies are useful in evaluating the significance of the parameters pertinent to thermal performance and in selecting the most appropriate HPHE for a particular application. Taking into account previous studies, the present work was focused on comparative studies between heat pipe and thermosyphon, at the same operating conditions, with following the objectives : • Further studying the heat transfer characteristics of heat pipe and thermosyphon to obtain additional information regarding thermal performance, at the same operating conditions. • Providing some guide-lines for HPHEs design which relate to the thermal performance of heat pipe and thermosyphon. To achieve these objectives, two computer models, a ‘Heat Pipe and Thermosyphon Thermal Performance Model’ and a ‘Heat Pipe Heat Exchangers Design Model’, were developed. The former model facilitates the prediction of the axial temperature distributions of the working fluid and the tube wall ; heat transport limitations; and the thermal performances of thermosyphons and heat pipes with different type of wicks. An experimental investigation of the steady-state behaviour of the thermosyphon was carried out to validate this computer model and the results were compared with the corresponding predictions of the computer model developed. According to the experimental results, Nusselt’s film theory for continuous liquid film could not be employed for predicting thermal performances in the condenser and evaporator film region of inclined thermosyphons. Liquid film could only be maintained circumferentially in vertical thermosyphon tubes, even though might not the smooth continuous film. In the evaporator pool region, the change in saturation temperature due to the hydrostatic height of the pool should be considered for predicting the axial temperature distributions. The results of the behaviours of heat pipes and thermosyphons were utilised to develop a ‘Heat Pipe Heat Exchangers Design Model’ to facilitate both the design and the prediction of performances of HPHEs. These models enable the designers of waste-heat recovery systems to choose the most appropriate type of heat-transfer elements and optimise the design parameters of a HPHE for a given application.