Development and Selection of Gas-Liquid High Pressure Heat Exchangers for External Combustion Heat driven Heat Pumps

Abstract

Vuilleumier heat pumps (VHPs) are the heat energy dri ven heat pumps that work on the Vuilleumier cycle. The Vuilleumier heat pump is a closed system that captures heat at l ower temperatures and deposits the captured heat energy to some intermediate temperature at the expense of high temperature hea t energy. The heat energy is the input to the system by burning fuel externally. Due to the interfacing between the heat pump and the outside heat sources and heat sinks, heat exchangers represent critical components in the VHP and have a major impact on overall coefficient of performance. High operating pressures, minimal dead volume and mini mal pressure drop are several key challenges that need to be addressed during the downselection of heat exchangers for Vuilleumier heat pumps. This paper focuses on the anal ysis and configurati on selection of the VHP heat exchangers using simplified 2D models and tailored analysis techni ques to eval uate the performance of the heat exchangers. Cross-flow heat exchangers are generally well suited for their application in VHPs because of their robust structure and as they are compact in size. Because the flui d flow i n cross flow heat exchangers predominantly covers all three di mensions, simulating the flui d fl ow in 3D becomes an integral part of CFD modelling and thermal anal ysis. 3D CFD models are particul arly computati onal intensi ve due to the invol vement of l arge number of elements in the mesh. Appropri atel y developed 2D models can replicate 3D models efficiently in many ways (particul arly for cross flow heat exchangers).They tend to save consi derable computational time and can also maintai n good levels of accuracy. The appropriate simplifying of 3D models, however, require good approxi mation techni ques and use of anal ytical and numerical methods to converge on a soluti on. The literature describes efficient techni ques of blendi ng anal ytical and numerical methods to deri ve the solution. Nomenclature= Nusselt Nu mber for internal d iameter D, = Reynolds number for the flow of gas through the pipe with diameter D. = Prandtl nu mber of the fluid = 0.3 for cooling of the fluid and 0.4 for heating of the flu id. = Specific heat at constant pressure for heliu m gas.(J/KgK) = Thermal conductivity of the gas (W/mK) = Density of fluid = Velocity of fluid flo w (heliu m gas flow in let velocity) (m/s) = Fluid viscosity at the bulk fluid temperature = Fluid v iscosity at the heat transfer boundary surface temperature. = Pressure drop for the fluid flo w through pipe (Pa), = Darcy's friction factor, L = Length of the tube, D = Internal diameter of the tube, 1 Student, Mechanical Eng ineering Depart ment, yogeshbedekar1@gmail.com and Student Member. Downloaded by UNIVERSITY OF TENNESSEE on August 8, 2015 | http://arc.aiaa.org | 2 Figure I-1-Schematic Diagram of VHP = Heat energy being picked up by the water, = Specific heat at constant pressure for water, = Difference in temperature of outlet and inlet.