An experimental study on flow boiling pressure drop in multi-microchannel evaporators with different refrigerants

A key parameter in designing two-phase flow systems for the cooling of high heat flux electronics is the pressure drop in microchannel evaporators. As a result, an experimental study was performed to investigate the flow boiling pressure drop of refrigerants in two silicon multi-microchannel evaporators. Three types of refrigerants (R1233zd(E), R245fa and R236fa) were tested under three inlet subcoolings and three nominal outlet saturation temperatures. The test section's backside base temperatures were measured by an infrared (IR) camera. A single-phase flow validation in terms of the inlet and outlet restriction pressure drops and the channel flow friction factor was carefully done before the boiling tests. The operating conditions for stable flow boiling tests were: mass fluxes from 1250 to 2750 kg m(-2) S-1, heat fluxes from 20 to 64 W cm(-2). The resulted maximum vapor quality at the outlet manifold was 0.51. It is found that within the present test conditions the channel pressure drop increased with the inlet subcooling and inlet orifice width but slightly affected by the outlet saturation temperature. In addition, compared to the other two types of refrigerants, R236fa exhibited the lowest channel pressure drop due to its smallest liquid to vapor density ratio and liquid viscosity. Based on the obtained 184 points of stable flow boiling data, a new empirical model suitable for the high mass flux operating conditions was developed. The new pressure drop model yielded the best prediction of the experimental data with a mean absolute error (MAE) of 27.8% and it was thus implemented to predict the local pressure and temperature profiles, allowing a quantitative analysis to obtain highly accurate local heat transfer coefficients. (C) 2016 Elsevier Inc. All rights reserved.

Flow Boiling Pressure Drop and Heat Transfer of Refrigerants in Multi-microchannel Evaporators under Steady and Transient States

Houxue Huang

Multi-microchannel evaporators used for the cooling of high heat flux electronics have been of interest to both industry and academia for more than a decade. Such interest has sparked a large number of research studies on the flow boiling pressure drop and heat transfer in multi-microchannel evaporators. However, there are still several aspects that need to be addressed in order to better understand the complicated flow boiling process taking place in such micro-evaporators. Firstly, the mechanism governing flow boiling heat transfer in microchannels is arguable; secondly, the availability of fine-resolution local heat transfer data is very limited; thirdly, over-simplified heat conduction models are used in the literature to reduce such local heat transfer data; finally, rare attention has been taken on the thermal behavior of such micro-evaporators under transient status.\ Inspired by the forgoing aspects, an extensive experimental program has been conducted to study the flow boiling pressure drop and heat transfer of refrigerants in multi-microchannel evaporators under steady and transient status. For the steady-state tests, three fluids ( R245fa, R236fa and R1233zd(E)) were tested in two multi-microchannel evaporators. The silicon microchannel evaporators were 10 mm long and 10 mm wide, having 67 parallel channels, each 100

\times

100

\mu

^2

, separated by a fin with a thickness of 50

\mu m

. Two types of micro-orifices (25 and 50

\mu

^2

in width) were placed at the entrance of each channel to stabilize the two-phase flow and to obtain good flow distribution. The test section backside temperatures were measured by a self-calibrated infrared (IR) camera. The operating conditions for stable flow boiling tests were: mass fluxes from 1250 to 2750 kg~m

^{-2}

^{-1}

, heat fluxes from 20 to 64 W cm

^{-2}

, inlet subcoolings of 5.5, 10 and 15 K, and nominal outlet saturation temperatures of 31.5, 35 and 40

^{\circ}\mathrm{C}

. The resulting maximum exit vapor quality at the outlet manifold was 0.51. \ The steady-state experimental data were reduced by solving a 3D inverse heat conduction problem to obtain the local heat transfer coefficients on a pixel-by-pixel basis. The required fluid temperature in the subcooled region was calculated from the local energy balance, while that in the saturated flow boiling region came from the general pressure drop model proposed in this manuscript based on the present data base. According to the present data base of fine-resolution local heat transfer coefficients, a new flow pattern based prediction model was developed here starting from the subcooled region all the way through the annular flow regime. This new flow pattern based model predicted the total local heat transfer database (1,941,538 local points) well with a MAE of 14.2% and with 90.1% of the data predicted within

\pm

30%. It successfully tracks the experimental trends without any jumps in predictions when changing flow patterns.\ For the transient tests, an extensive experimental study was conducted to investigate the base temperature response of multi-microchannel evaporators under transient heat loads, including cold startups and periodic step variations in heat flux using two different test sections and two coolants (R236fa and R245fa) for a wide variety of test conditions. In addition, a transient flow boiling test under a heat flux disturbance was performed, and a new method of solving the transient 3D inverse heat conduction problem was proposed to obtain the local transient flow boiling heat transfer coefficients.

EPFL2016

Two-phase flow of refrigerants in 85μm-wide multi-microchannels: Part II – Heat transfer with 35 local heaters

Etienne Costa-Patry, Bruno Michel, Jonathan Olivier, John Richard Thome

This article is the second part of a study on flow boiling of R236fa and R245fa. This part presents the heat transfer coefficients obtained in a 12.7 mm silicon evaporator composed of 135 microchannels with 85 mu m wide and 560 mu m high channels separated by 46 mu m wide fins. There were 35 local heaters and temperature measurements arranged in a 5 x 7 array. The heat transfer results were uniform in the lateral direction to the flow (attributable to the inlet restriction) and a function of the heat flux, vapor quality and mass flux. The steady-state standard deviation of the local base temperature was less than 0.2 degrees C, inferring that the boiling process was very stable. For wall heat fluxes over 45 kW/m(2), the heat transfer coefficient curves were V-shaped, decreasing for intermittent flow regimes and increasing for annular flow. The three-zone model of Thome et al. (2004) was the best heat transfer prediction method when setting the dryout thickness equal to the channel roughness. (c) 2011 Elsevier Inc. All rights reserved.

Elsevier2011

Flow Boiling of R134a in a Multi-Microchannel Heat Sink with Hotspot Heaters for Energy-Efficient Microelectronic CPU Cooling Applications

Etienne Costa-Patry, Yassir Madhour, Bruno Michel, Jonathan Olivier, John Richard Thome

This paper focuses on two-phase flow boiling of refrigerant R134a inside a copper multi-microchannel heat sink for microelectronic central processing unit cooling applications. The heat sink is composed of 100 parallel microchannels, 100 mu m wide, 680 mu m high, and 15 mm long, with 72-mu m-thick fins separating the channels. The base heat flux was varied from 2.57 to 189 W/cm(2) and the mass flux from 205 to 1000 kg/m(2)s, at a nominal saturation temperature of 63 degrees C. Over 40 000 local heat transfer coefficients were measured at 35 locations using local heaters and temperature sensors, for which different heat transfer trends were identified. The main ones were that the heat transfer coefficient increased with heat flux and was independent of mass flow rate. Heat transfer coefficients as high as 270 000 W/m(2)K (relative to the base area) were reached, keeping the chip under 85 degrees C with a maximum of 94 kPa of pressure drop, for no inlet subcooling and a coolant flow rate of 1000 kg/m(2)s.

2011