Variable geometry microchannel heat exchanger modeling under dry, wet, and partially wet surface conditions accounting for tube-to-tube heat conduction

Abstract

Numerical models for microchannel heat exchangers (MCHX) are favored in research and development process due to their cost effectiveness as opposed to prototype development and testing. A literature survey suggests that there lacks a unified air-to-surface heat and mass transfer modeling approach for MCHX, especially under dehumidifying conditions with tube-totube heat conduction. In this research, we present an air-to-fin heat and mass transfer model for MCHX operating under dry, wet and partially wet conditions. Typically, there are two boundary conditions for the fins in MCHX. The adiabatic fin tip boundary condition is applied to the extended fins on top and bottom of the microchannel slab. The second boundary condition is the prescribed surface temperature, applicable to a fin bounded by two tubes. The proposed fin Downloaded by [New York University] at 21:58 24 June 2015 ACCEPTED MANUSCRIPT ACCEPTED MANUSCRIPT 2 analysis method accounts for both boundary conditions and tube-to-tube conduction. The modeling approach is capable of locating the boundary between dry and wet surface if a fin is partially wet. The model is verified against simulation results for air-to-surface heat transfer on a fin obtained using a commercially available Computational Fluid Dynamics (CFD) package. A new finite volume MCHX model is developed using the proposed fin analysis method. The model is capable of predicting the performance of a variable geometry MCHX (VG-MCHX) under both dry and dehumidifying conditions, and is validated against experimental data. The average absolute capacity deviation between the predicted and measured values is 2.44% for condensers and 2.92% for evaporators. The absolute difference in sensible heat ratio for evaporators between predicted and measured values is 0.018. The proposed model allows for the most comprehensive and accurate analysis of microchannel evaporators and condensers. Nomenclature A surface area [m 2 ] Cp specific heat [J·kg -1 ·K -1 ] CG-MCHX conventional geometry microchannel heat exchanger h enthalpy [J·kg -1 ] k thermal conductivity [W·m -1 ·K -1 ] L fin height [m] m & mass flow rate [kg·s -1 ] MCHX variable geometry microchannel heat exchanger NTU number of transfer units [dimensionless] P perimeter [m] Q heat transfer rate [W] T temperature [K] U heat transfer coefficient [W·m -2 ·K -1 ] VG-MCHX variable geometry microchannel heat exchanger Dx distance [m] Greek letters a local heat transfer coefficient [W·m -2 ·K -1 ] a d local mass transfer coefficient [m·s -1 ] Downloaded by [New York University] at 21:58 24 June 2015 ACCEPTED MANUSCRIPT ACCEPTED MANUSCRIPT 4 q excess temperature [K] w humidity ratio [kg vapor · kg dry air -1 ] e effectiveness [dimensionless]

Two-dimensional numerical modeling for the air-side of minichannel evaporators accounting for partial dehumidification scenarios and tube-to-tube heat conduction

“…Currently, several models or simulation tools for heat exchangers are available in the literature: for conventional-channel (Liang et al, 2001;Corberán et al, 2002;Domanski, 2003;Jiang et al, 2006;Singh et al, 2008;IMST-ART, 2010); and for minichannel (Asinari et al, 2004;Gossard et al, 2013;Huang et al, 2015;Martínez-Ballester et al, 2011;Ren et al, 2013;Wu and Webb, 2002;Yin et al, 2015;Zhao et al, 2012).…”

A new moving boundary model for evaluating the performance of wet fins: Application to minichannel evaporators

“…Huang et al [7] proposed an air-to-fin heat and mass transfer minichannel evaporator model. This model accounts for variable geometry minichannel heat exchangers, partial dehumidification conditions, and tube-totube heat conduction.…”

Development and Validation of a Minichannel Evaporator Model under Dehumidification

“…Currently, several models or simulation tools for heat exchangers are available in the literature: for finned conventional-tubes (Domanski, 1991;Ragazzi, 1995;Lee and Domanski, 1997;Corberán et al, 2002;Jiang et al, 2006;Singh et al, 2008;CoilDesigner, 2010;EVAP-COND, 2010;IMST-ART, 2010) and for micro/minichannel heat exchangers (Kim and Bullard, 2001;Yin et al, 2001;Wu and Webb, 2002;Asinari et al, 2004;Fernando et al, 2008;Brix et al, 2009;García-Cascales et al, 2010;Jin et al, 2011;Martínez-Ballester, 2012;Zhao et al, 2012;Gossard et al, 2013;Ren et al, 2013;Huang et al, 2014;Huang et al, 2015;Yin et al, 2015).…”

“…Some of the above mentioned models (Kim and Bullard, 2001;Yin et al, 2001;Corberán et al, 2002;Asinari et al, 2004;Fernando et al, 2008;Singh et al, 2008;Brix et al, 2009;CoilDesigner, 2010;IMST-ART, 2010;Jin et al, 2011;Martínez-Ballester, 2012;Ren et al, 2013;Huang et al, 2014;Huang et al, 2015) apply the energy conservation equations to each control volume or finite element, while others (Domanski, 1991;Ragazzi, 1995;Lee and Domanski, 1997;Wu and Webb, 2002;Jiang et al, 2006;EVAP-COND, 2010;García-Cascales et al, 2010;Zhao et al, 2012;Gossard et al, 2013;Yin et al, 2015) apply the classical ε-NTU method. The main difference between both methodologies is that the ε-NTU method uses several implicit assumptions resulting in less freedom to describe the actual phenomena, especially at a presence of the dehumidification process on the air-side of the evaporator.…”

“…Many of evaporator models (Ragazzi, 1995;Jiang, 2003;Huang et al, 2015) employ the McQuiston's constant (Equation 1.19) within the whole evaluated evaporator segment. This constant is usually evaluated at the fin base condition or calculated as an average value between fin base and tip conditions (Mirth and Ramadhyani, 1993).…”