Experimental investigation on flow boiling pressure drop and heat transfer of R1233zd(E) in a multi-microchannel evaporator

An experimental study on flow boiling pressure drop and heat transfer of a new environmentally friendly refrigerant R1233zd(E), in a parallel multi-microchannel evaporator was carried out. The silicon microchannels evaporator was 10 mm long and 10 mm wide, having 67 parallel channels, each 100 x 100 mu m(2), separated by a fin with a thickness of 50 mu m. Upstream of each channel, a micro orifice was placed to stabilize the two-phase flow and to obtain good flow distribution. The operating conditions for flow boiling tests were: mass fluxes from 500 to 2750 kg m(-2) S-1, heat fluxes from 6 to 50 W cm(-2), inlet subcooling of 5.8 K, and a nominal outlet saturation temperature of 35 degrees C for stable flow boiling. The test section's backside base temperatures were measured by an infrared (IR) camera. The stable flow boiling data without back flow was selected through flow visualization recorded by a highspeed camera coupled with a microscope. These data were then used to assess the applicability of existing two-phase pressure drop models, and to further develop a new empirical model suitable for the high mass flux operating conditions. This new pressure drop model was used to predict the local fluid temperature for the further heat transfer data identification. The fine-resolution local heat transfer coefficients were obtained by solving the three-dimensional inverse heat conduction problem. The experimental results showed that in the saturated flow boiling region the local heat transfer coefficient first decreased moderately in the very low vapor quality region (x < 0.05), then increased significantly but monotonically along the flow direction. The fine-resolution local heat transfer data at the saturated flow boiling region were compared with two groups of heat transfer correlations. The first one considered the flow boiling mechanism occurring in muliti-microchannels as a combination of nucleate boiling and forced convection boiling, while the other one associated this mechanism to liquid thin film evaporation, thus indicating a controversy. It is found that the flow pattern based model belonging to the second group yielded the best agreement with the experimental data, predicting 92.0% of this new database within 30%. (C) 2016 Elsevier Ltd. All rights reserved.

Two-phase operational maps, pressure drop, and heat transfer for flow boiling of R236fa in a micro-pin fin evaporator

Navid Borhani, Chiara Falsetti, Hamideh Jafarpoorchekab, Mirco Magnini, John Richard Thome

Cooling the new generation of 3D high power electronic chips is one of the leading challenges in microelectronics, as it is a key to achieve high computational performance at lower cooling system power consumption, thus reducing the operating cost. Two-phase flow boiling in two micro-pin fin heat sinks is studied here for this cooling process. Each micro-evaporator has a heated area of 1 cm(2) and contains 66 rows of cylindrical micro-pin fins with an in-line configuration and diameter, height and pitch of respectively 50 mu m,100 gm and 91.7 mu m. The fluid tested is refrigerant R236fa. Channel entrances with and without inlet restrictions are tested in order to evaluate their relative effect on the stability of the flow. The inlet restrictions consist of an extra row of micro-pin fins with a larger diameter of 100 mu m placed at the inlet of the heated area. The present study investigates operational stable and unstable flow regimes, pressure drop and heat transfer performance for the two micro-evaporators (one with and one without inlet restrictions) by coupling high speed visualization of the micro-pin fins flow area with fine resolution infrared temperature measurements of the micro-evaporator base, which yields a 2D map of local heat transfer coefficients of the whole heated area. Working conditions tested have mass flux varying from 500 kg m(-2) S-1 to 2500 kg m(-2) S-1, heat flux ranging from 20 W cm(-2) to 48 W cm(-2), and a constant outlet saturation temperature of 30.5 degrees C. In agreement with previous studies for multi-microchannel evaporators, it is here observed that inlet restrictions extend the map of stable operational regimes, in particular toward lower values of the mass flux, without any appreciable increase of the pressure drop. Unlike evaporation through parallel microchannels, the heat transfer coefficient trends and magnitudes vary dramatically with the flow conditions (mass flux and heat flux), thus suggesting that micro-pin fins have a strong impact on the two-phase flow pattern development, in particular delaying the transition to annular flow. (C) 2016 Elsevier Ltd. All rights reserved.

Pergamon-Elsevier Science Ltd2017

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

Boiling on a tube bundle

Francesco Agostini

The complexity of the two-phase flow in a tube bundle presents important problems in the design and understanding of the physical phenomena taking place. The working conditions of an evaporator depend largely on the dynamics of the two-phase flow that in turn influence the heat exchange and the pressure drop of the system. A characterization of the flow dynamics, and possibly the identification of the flow pattern in the tube bundle, in thus expected to lead to a better understanding of the phenomena and to reveal on the mechanisms governing the tube bundle. Therefore, the present study aims at providing further insights into two-phase bundle flow through a new visualization system able to provide for the first time a view of the flow in the core of a tube bundle. In addition, the measurement of the light attenuation of a laser beam through the two-phase flow and measurement of the high frequency pressure fluctuations with a piezo-electric pressure transducer are used to characterize the flow. The design and the validation of this new instrumentation also provided a method for the detection of dry-out in tube bundles. This was achieved by a laser attenuation technique, flow visualization, and estimation of the power spectrum of the pressure fluctuation. The current investigation includes results for two different refrigerants, R134a and R236fa, three saturations temperatures Tsat = 5, 10 and 15 °C, mass velocities ranging from 4 to 40 kg/sm2 in adiabatic and diabatic conditions (several heat fluxes). Measurement of the local heat transfer coefficient and two-phase frictional pressure drop were obtained and utilized to improve the current prediction methods. The heat transfer and pressure drop data were supported by extensive characterization of the two-phase flow, which was to improve the understanding of the two-phase flow occurring in tube bundles.

EPFL2008

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

Houxue Huang