The airside thermal resistance of air-to-fluid heat exchangers dominates the overall thermal resistance. On conventional heat exchanger's, fins are required to address such challenges; but their benefits are not limitless and are bound mainly by the tube size and shape. The reduction of the tube characteristic length has favorable impact on compactness and heat transfer. Conventional tubes are typically limited to round, elliptical or flat shapes which result in particular thermal-hydraulic characteristics. The current article has three main objectives. First, discuss the importance of fins on typical air-to-fluid heat exchanger's and how they become unattractive at smaller characteristic lengths with numerical analyses to support this argument from different perspectives. Second, present a proof-of-concept design with small finless tubes and a novel shape that can outperform a microchannel heat exchanger. Third, present a comprehensive analysis with shape optimization leveraging automated computational fluid dynamics simulations and approximation assisted optimization techniques. Optimum designs can achieve more than 50% reduction in size, material, and pressure drop compared to the baseline microchannel heat exchanger. The method is validated with the experimental validation of a metal three-dimensional printed prototype of the NTHX-001. The numerical simulations agreed within less than 5% in capacity, 10% in air heat transfer coefficient, and 15% in air pressure drop.

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]

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This article presents new equations for airside friction and heat transfer characteristics for bare tube air-to-refrigerant heat exchangers with tube diameters ranging from 0.5mm to 2mm. There are no airside correlations for such small diameter tubes in the literature. Furthermore, conventional empirical correlation development relies on testing of samples, which is inherently time consuming, expensive and has a limited range of applicability. The correlations presented in this article are developed based on comprehensive CFD simulations for a large design space and include experimental validation. The proposed correlations predict more than 80% of source data within 10% error and more than 90% of the source data within 20% error. The proposed correlations are valid for 2 to 40 rows of tubes.

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