A novel time strip flow visualisation technique for investigation of intermittent dewetting and dryout in elongated bubble flow in a microchannel evaporator

A flow visualisation study of flow boiling of R245fa in silicon multi-microchannels at low mass flux and moderate heat flux has been carried out with a high speed digital camera. The micro-evaporator had 67 channels of length 20 mm, width 223 mu m, and height 680 mu m while the fin width between adjacent channels was 80 mu m. The base heat flux ranged from 2 to 26W cm(-2) for a mass velocity of 100 kg s(-1) m(-2), resulting in exit vapour qualities ranging from 10% to 70%. In particular, a novel time strip technique was developed to analyse the recorded image sequences and significantly highlight the various phenomena occurring along given channels. Notably, this technique was able to reveal profound details regarding the intermittent dryout mechanism of liquid films trapped between the elongated bubbles and the heated channel walls. The results show that the intermittent dryout of the evaporating liquid film is comprised of four stages with distinct time scales and dynamics: (i) the growth of liquid film thinning perturbations to a critical amplitude causing the rupture of the metastable liquid film, (ii) a dewetting stage involving expanding dry spots leading to a rivulet flow regime, (iii) evaporation of the rivulets leading to full dryout, and (iv) a rewetting stage. This intermittent dryout mechanism appears to explain the many seemingly contradictory heat transfer coefficient trends observed with changes in vapour quality in microchannels, thus resolving an important heat transfer dilemma. Furthermore, since dryout is an undesirable event during the practical application of a microchannel evaporator, it is important to delay or even suppress the initial rupture of the liquid film that leads to dryout. This can be achieved by manufacturing or treating the channel surfaces to be highly wettable with the chosen refrigerant. (C) 2010 Elsevier Ltd. All rights reserved.

Contribution to the heat transfer analysis of natural and substitute refrigerants evaporated in a horizontal tube

Olivier Zürcher

The substitution of CFC refrigerants in refrigeration systems, heat pumps and organic Rankine cycles, requests a good knowledge of the heat transfer and pressure drop properties of substitute fluids. A contribution to this international effort is proposed with the study of two hydrofluorocarbon refrigerants (HFC-134a and the zeotropic mixture HFC-407C) and the study of the natural refrigerant ammonia. The HFCs have been experimentally tested on the first test rig developed in the Laboratory of Industrial Energy Systems (LENI) which was substantially modified to cope with ammonia, both in terms of safety requirements and operational conditions. The experimental test section is composed of two concentric tubes, with in tube evaporation of the refrigerant inside the inner tube of counter-current Annular water heating. A new database of the local heat transfer coefficients for the refrigerants HFC-134a, HFC-407C and ammonia together with pressure drop measurements has been collected and used to define and calibrate a new and more general heat transfer model. The two HFC refrigerants have been tested on the test rig developed by Kattan [36]; due to the chemical and the thermophysical properties of ammonia, a new test rig has been designed for the ammonia tests and a new calculation procedure based on the mean temperature measurement on the tube wall has been used. The experimental database covers evaporation tests on plain and on microfin tubes, and with nominal oil concentrations varying from 0 to 5 [%] (by wt.). Detailed modeling is concentrated on the tests on plain tube without oil. Each of the three refrigerants has been evaporated at 4[°C]. The other experimental conditions for the HFCs refrigerants were an inside tube diameter of 10.92[mm], a heat flux range of 2 – 5 [kW/m2] and three mass velocities of G = 100, 200, 300 [kg/(m2s)] visualized as Stratified-Wavy flows (G = 100[kg/(m2s)]), and mainly Annular for the higher mass velocities (G = 200, 300[kg/(m2s)]). The other global conditions for ammonia were an internal tube diameter of 14.00 [mm], a heat flux range of 5 – 70[kW/m2] and eleven mass velocities of G = 10,20,30,40,45,50,55,60,80,120,140[kg/(m2s)], corresponding to Stratified, Stratified-Wavy, Intermittent and Annular flow patterns; complementary tests, varying the mass velocity at constant vapor qualities (x = 20,50,80[%]), have been made in particular to better characterize boundaries between flow patterns. The flow patterns are visualized through glass sections at both ends of the 3.1 [m] long test tube. An extensive review of void fraction models with sensitivity analysis on the actual available flow pattern map has been made. From the 17 correlations reviewed, the models of Taitel-Dukler [71] and Rouhani [79] have been used. Other improvements to the threshold criteria of the Stratified-Wavy to Annular transition, lead to an accurate diabatic map to predict the flow patterns and their transition for very different families of fluids like HFC's and ammonia. A new approach in the prediction of two-phase flow heat transfer has been proposed through the study of each flow pattern separately, according to a new criterion defining the onset of nucleate boiling as a function of the critical convective heat transfer coefficient representative of the location where nucleate boiling might occur. A function based on a pseudo Biot number allows, from the mean heat flux around the tube periphery, to base the model on two different mean heat fluxes applied respectively to the parts of the perimeter in contact with the liquid and the vapor in stratified types of flow. Considering pure convective heat transfer, or mixed convective and nucleate heat transfer, this division allows the use of a common criterion to be applied to each flow pattern. The two-phase flow heat transfer coefficient has finally been obtained as a weighted mean function of the vapor and the liquid heat transfer coefficient with respect to their contact surface with the heated tube. Based on the database of refrigerants HFC-134a and ammonia, the standard deviation of the new heat transfer model is of σ = 27.9[%]. Even if the database showed that the flow conditions were close to, or in the turbulent to laminar flow transition, and even if the major part of the experimental points were purposely obtained close to the various flow pattern transitions, the new model showed very good agreement with the experimental database. Due to the precision of the new flow pattern map and the effectiveness of the onset on nucleate boiling criterion, this new heat transfer model accurately predicts the heat transfer conditions during evaporation. Finally, a new method to model separated flows is proposed, based on partial hydraulic diameters and a mean interface velocity.

EPFL2000

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

Navid Borhani, Houxue Huang, John Richard Thome

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.

Pergamon-Elsevier Science Ltd2016

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