Multi-microchannel Evaporators Research Articles

Experimental analysis of flow boiling heat transfer in multi-microchannel evaporators

This paper provides an experimental analysis of flow boiling heat transfer in four finned evaporators with different aspect ratio and heated length have been tested. R245fa was the refrigerant chosen as working fluid for its good thermal characteristics. The imposed heat flux at the footprint covered a range from 30 to 330 kW/m2, while the fluid mass fluxes within the channel varied in a range between 17 and 225 kg/m2 s. The hydraulic diameters of the investigated evaporators were 1.15 mm and 2.09 mm, with aspect ratio (channel width/channel height) respectively of 0.3 and 0.72. Thanks to the measuring apparatus, heat transfer data were collected and then processed to calculate the local heat transfer coefficients, using the fin array heat transfer model. These local values of the heat transfer coefficient were then compared with those calculated from existing correlations for mini-micro channels flow boiling and the Mean Absolute Percent Error was finally assessed.

Computational study of bubble, thin-film dynamics and heat transfer during flow boiling in non-circular microchannels

Flow boiling in multi-microchannel evaporators is one of the most efficient thermal management solutions for high-power-density applications. However, there is still a lack of understanding of the governing two-phase heat and mass transfer processes that occur in these devices, which has resulted in a limited availability of applicable boiling heat transfer prediction methods based on first principles, and of reliable thermal design tools. This article presents a systematic analysis of the dynamics of bubbles and the surrounding liquid film during flow boiling in three-side-heated non-circular microchannels. The study is performed using a custom version of ESI OpenFOAM v2106 with a geometric volume-of-fluid method to capture the interface dynamics, also incorporating conjugate heat transfer through the evaporator walls. The hydraulic diameter of the channel is fixed to Dh=0.229mm and the range of width-to-height aspect ratios ϵ=0.25−4 is examined. We investigate different fluids, namely water, HFE7100, R1233zd(E), R1234ze(E), and evaporator materials, namely copper, aluminium, silicon, stainless steel, with base heat fluxes in the range qb=50−200kW/m2. The results show that conjugate heat transfer acts to make the temperature distributions around the perimeter of the channel cross-section more uniform, and that the topography of the lubricating film and the extension of the dry vapour patches that develop while the film is depleted both depend on the cross-sectional channel shape and influence the heat transfer performance significantly. For highly wetting conditions, channels with ϵ=0.25 tend to allow enhanced heat transfer rates, with a spatially-averaged Nusselt number that is 50% higher than that obtained for ϵ=1 (square channels) and 10% higher than that for ϵ=4. This arises thanks to an extended evaporating film that covers the vertical walls which, owing to the three-side-heated configuration, contribute twice to the spatially-averaged heat transfer performance. For more hydrophobic conditions, large dry patches develop over the vertical walls for ϵ=0.25 due to the lower evaporator temperatures, leading to reduced heat transfer, with thermal performance weakly dependent on ϵ in the range ϵ=0.5−2.

High Heat Flux Cooling Technologies Using Microchannel Evaporators: Fundamentals and Challenges

Flow boiling in multi micro-channel evaporators is a promising approach to cooling high heat flux in many devices. Understanding of the fundamentals of flow boiling in multi microchannel evaporators is urgently needed. This paper presents a critical review on the research of flow boiling phenomena in microchannels. First, classifications of macro- and micro-channels are briefed. Then, studies on flow boiling in multi microchannels with plain surface are analyzed. Next, studies of flow boiling in multi microchannels with various enhanced technologies are analyzed. Finally, heat transfer mechanisms and development of prediction methods are discussed. According to the critical review and analysis of the current research on the fundamental issues and challenges of flow boiling in micro-channels, future research needs have been identified and recommended. In general, systematic, and accurate experimental data of flow boiling in micro-channels are needed by using advanced measurement technologies. In particular, optimal design of microchannel evaporators with plain surface and enhanced structures is lacking and should be investigated. The physical mechanisms of flow boiling should be further investigated based on the corresponding flow patterns. Furthermore, mechanistic prediction methods should be developed. Furthermore, systematic experimental, analytical and modeling studies on transient flow boiling phenomena in micro-channels should be conducted to understand the physical mechanisms and develop the theoretical models.

Conjugate heat transfer effects on flow boiling in microchannels

• Heat distributes nonuniformly around the perimeter of microchannels in evaporators. • In single-phase flow, low aspect-ratio channels yield best heat transfer. • In two-phase flow, the Nusselt number trends vs. aspect-ratio are mixed. • High aspect-ratio channels exhibit larger thermal resistance and higher evaporator temperatures. • Conjugate heat transfer should be included in design models for evaporators. This article presents a computational study of saturated flow boiling in non-circular microchannels. The unit channel of a multi-microchannel evaporator, consisting of the fluidic channel and surrounding evaporator walls, is simulated and the conjugate heat transfer problem is solved. Simulations are performed using OpenFOAM v2106 and the built-in geometric Volume Of Fluid method, augmented with self-developed libraries to include liquid-vapour phase-change and improve the surface tension force calculation. A systematic study is conducted by employing water at atmospheric pressure, a channel hydraulic diameter of D h = 229 µm, a uniform base heat flux of q b = 100 kW / m 2 , and by varying the channel width-to-height aspect-ratio and channel fin thickness in the range ϵ = 0.25 –4 and W f = D h / 8 − D h , respectively. The effects of conjugate heat transfer and channel aspect-ratio on the bubble and evaporative film dynamics, heat transfer, and evaporator temperature are investigated in detail. This study reveals that, when the flow is single-phase, higher Nusselt numbers and lower evaporator base temperatures are achieved for smaller channel aspect-ratios, from Nu ≃ 4 and T b − T s a t ≃ 9 K when ϵ = 4 , to Nu ≃ 6 and T b − T s a t ≃ 2 K when ϵ = 0.25 , for same fin thickness W f = D h / 8 . In the two-phase flow regime, Nusselt numbers in the range Nu = 12 − 36 are achieved. The trends of the Nusselt number versus the aspect-ratio are non-monotonic and exhibit a marked dependence on the channel fin thickness. For small fin thicknesses, W f = D h / 8 and W f = D h / 4 , an overall ascending trend of Nu for increasing aspect-ratios is apparent, although in the narrower range ϵ = 0.5 –2 the Nusselt number appears weakly dependent on ϵ . For thicker fins, W f = D h / 2 and W f = D h , the Nusselt number decreases slightly when increasing the aspect-ratio in the range ϵ = 0.5 –2, although this trend is not monotonic when considering the entire range of aspect-ratios investigated. Nonetheless, due to conjugate heat transfer, Nusselt numbers and evaporator base temperatures follow different trends when varying the aspect-ratio, and channels with ϵ < 1 seem to promote lower evaporator temperatures than higher aspect-ratio conduits.

Air-Cooled Loop Thermosyphon Cooling System for High Heat Load CPUs—Part I: Design and Performance Simulation

This article proposes and evaluates the thermal-hydraulic performance of a compact, lightweight, and high heat-transfer capacity cooling system for central processing unit (CPU) temperature control. The cooling loop, defined as a closed loop thermosyphon (LTS), has a passive flow driven mainly by the buoyancy in the riser. It is a self-modulated system, i.e., without heat flow ceases but it increases with increasing heat load. The LTS is a partially passive system since energy is still required to drive the fan blowing air through its condenser. The main advantage relative to conventional forced air-cooled heat spreaders is its ability to transfer much higher levels of heat while consuming less fan power. The innovative patented LTS system has two thermosyphon loops working in parallel, composed of a multimicrochannel evaporator, a multiport tube louvered fin condenser, two risers, and one downcomer. The simulations show that it is able to cool 600 W from a 50 mm <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times $ </tex-math></inline-formula> 50 mm CPU footprint without dryout or surpassing the critical heat flux. It was specifically simulated for a CPU of 2U server, considering a height of only 66 mm and width of 214 mm. Thus, two of these units can be installed side-by-side in a 2U server, which means a total cooling capacity of 1200 W per server. Several simulations are presented showing the high thermal-hydraulic performance of the LTS and a case study showing the reduction of fan energy consumption when comparing the proposed LTS with an actual air-cooled heat spreader system. The LTS was prototyped, experimentally tested, and its simulations successfully validated in Part II of this study.

Peak Temperature Mitigation of a Multimicrochannel Evaporator Under Transient Heat Loads

Abstract Microchannel flow boiling has shown great cooling potential with steady-state studies demonstrating the capability to dissipate heat fluxes over 1 kW cm−2. However, most microelectronic devices undergo transient heat loads involving cold startups or pulse-like power operation. Transient heating events in low thermal resistance, low thermal capacity cold plates may exacerbate boiling instabilities and result in device damage or failure due to local dryout conditions. Currently, limited studies are investigating these effects and potential mitigation strategies. In this study, step function, or pulsed, and ramped heat loads are investigated on a multimicrochannel silicon evaporator using R134a under a range of heat fluxes and ramping rates. The transient temperature response of the base heater is recorded using a calibrated infrared (IR) camera, while fluid flow visualization is captured using a video camera microscope. Pulsed heat loads resulted in a large temperature overshoot in the test section until the fluid reached the onset of nucleate boiling (ONB), while significant vapor backflow is observed despite the presence of channel inlet restrictions. Steady boiling is eventually reached and vapor backflow is suppressed. The magnitude of the temperature overshoot is observed to be strongly dependent on peak heat flux. In contrast, ramped heat loads resulted in lower peak temperature rises before ONB as well as significantly reduced vapor backflow compared to the pulsed heat loads.

Experimental Analysis of the Condenser Design in a Thermosiphon System for Cooling of Telecommunication Electronics

Passive two-phase cooling systems are an efficient and viable alternative to conventional air-cooling schemes for thermal management of electronics due to their capability of dissipating higher power densities with lower power consumption and noise levels. This article aims to assess the performance of a novel two-phase air-cooled thermosyphon for cooling shelf-level telecommunications equipment that is designed with 18 line cards. In particular, the main objective is to investigate the effect of the condenser technology on the thermal-hydraulic performance of the cooling system. The thermosyphon consists of a multimicrochannel evaporator with 18 individual microchannel zones (one per card) connected to an air-cooled condenser via riser and downcomer tubes. The total height of the loop, from midevaporator to midcondenser, is approximately 50 cm. Two different condenser designs have been tested: a single-pass louvered-fin flat-tube condenser, and a multipass wavy-fin, circular-tube condenser. An extensive experimental campaign is performed to evaluate the performance of these two air-cooled condensers in a front-to-back airflow configuration. The experiments are carried out with R134a as the working fluid, varying filling ratios from 45% to 60%, and heat loads from 102 to 1023 W under uniform and nonuniform conditions. The influence of the air-side fan configuration is also investigated by varying the fans speed from 10 548 to 15 480 min−1 and by increasing the fan-tray height from 65 to 103 mm. The results demonstrate that the single-pass louvered-fin flat-tube condenser provides lower liquid-side frictional pressure drops and consequently higher refrigerant mass flow rate in the loop. This leads to a higher critical heat flux value, which is ideal for increasing power densities and limiting dry-out occurrence. On the other hand, the multiple-pass wavy-fin, condenser offers lower thermal resistances over a larger range of dissipated heat loads. This is mainly attributed to its lower air-side pressure drop.

Numerical study of the impact of the channel shape on microchannel boiling heat transfer

Flow boiling in multi-microchannel evaporators is recognised as one of the most efficient cooling solutions for high-performance electronics, and has therefore received increasing attention during the recent years. Despite the extensive literature, there is no general agreement yet about the effect of the channel cross-sectional shape on the boiling heat transfer performance, which results on a limited availability of thermal design guidelines and tools. This article presents the results of a systematic analysis of the impact of the channel shape on the bubble dynamics and heat transfer, under flow boiling conditions. Simulations are carried out using a customised version of OpenFOAM, and the Volume-Of-Fluid method is chosen to capture the liquid-vapour interface dynamics. A benchmark flow model is utilised, where a single isolated bubble is seeded at the channel upstream and transported by a liquid flow across the diabatic section, which is heated by a constant and uniform heat flux. Flow conditions that apply well to the flow boiling of water or refrigerant fluids in sub-millimetre channels at low heat flux ( ~ 10 kW/m2) are investigated, with cross-section width-to-height aspect-ratios ranging from 1 to 8, while the hydraulic diameter of the channel is fixed. This study emphasises that the heat transfer performances for different channel shapes are closely related to the perimetral distribution of the liquid film surrounding the very long bubbles. Square channels exhibit the highest heat transfer coefficients at low flow rates, due to a very thin liquid film that forms at the centre of the wall, but are more at risk of film dryout. High aspect-ratio rectangular channels may be beneficial at larger flow rates, as they promote the formation of an extended liquid film that covers up to 80 % of the cross-section perimeter. At larger aspect-ratios, the average heat transfer coefficient along the shorter wall becomes orders of magnitude smaller than the value detected along the longer wall, owing to a strong asymmetry in the liquid film perimetral distribution.

A new method for reducing local heat transfer data in multi-microchannel evaporators

Measuring the strong local variation in heat transfer coefficients in multi-microchannel evaporators is related to the inverse heat conduction problem (IHCP). As the local flow heat transfer coefficients change greatly in magnitude from single-phase liquid at the entrance to a peak in slug flow and then to a minimum at the transition to the onset of annular flow and finally a new substantial rise up to the outlet, a significant heat spreading occurs due to the heat transfer process itself, and this has to be accounted for when processing the data. Until now, IHCP has not been introduced in the experimental study of heat transfer in such evaporators when reducing local experimental data. In this paper, a new method for processing experimental local heat transfer data by solving the 3D IHCP is proposed. This method is then applied and validated using two sets of single- and two-phase flow experimental data obtained with infrared (IR) camera temperature measurements. The 14 400 raw pixel temperatures per image from the IR camera are first pre-processed by a filtering technique to remove the noise and then to smooth the data, where the IR camera has undergone a prior inhouse pixel by pixel insitu temperature calibration. Three filtering techniques (Wiener filter, spline smooth, and polynomial surface fitting) are compared. The polynomial surface fitting technique was shown to be more suitable for the current type of data set. Then the 3D IHCP is solved based on a finite volume method using the TDMA (Tridiagonal Matrix Algorithm) solver with a combination of Newton-Raphson iteration and a local energy balance method. Furthermore, the present 3D TDMA method (named as 3D TDMA) is compared with three other post-processing methods currently used in the literature, among which the present one is found to be more accurate for reducing the local heat transfer data in multi-microchannel evaporators.

Transient Data Processing of Flow Boiling Local Heat Transfer in a Multi-Microchannel Evaporator Under a Heat Flux Disturbance

Multi-microchannel evaporators are often used to cool down electronic devices subjected to continuous heat load variations. However, so far, rare studies have addressed the transient flow boiling local heat transfer data occurring in such applications. The present paper introduces and compares two different data reduction methods for transient flow boiling data in a multi-microchannel evaporator. A transient test of heat disturbance from 20 to 30 W cm−2 was conducted in a multi-microchannel evaporator using R236fa as the test fluid. The test section was 1 × 1 cm2 in size and had 67 channels, each having a cross-sectional area of 100 × 100 μm2. The micro-evaporator backside temperature was obtained with a fine-resolution infrared (IR) camera. The first data reduction method (referred to three-dimensional (3D)-TDMA) consists in solving a transient 3D inverse heat conduction problem by using a tridiagonal matrix algorithm (TDMA), a Newton–Raphson iteration, and a local energy balance method. The second method (referred to two-dimensional (2D)-controlled) considers only 2D conduction in the substrate of the micro-evaporator and solves at each time step the well-posed 2D conduction problem using a semi-implicit solver. It is shown that the first method is more accurate, while the second one reduces significantly the computational time but led to an approximated solution. This is mainly due to the 2D assumption used in the second method without considering heat conduction in the widthwise direction of the micro-evaporator.

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

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 1cm2 and contains 66 rows of cylindrical micro-pin fins with an in-line configuration and diameter, height and pitch of respectively 50μm,100μm and 91.7μm. 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 100μm 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 500kgm−2s−1 to 2500kgm−2s−1, heat flux ranging from 20Wcm−2 to 48Wcm−2, and a constant outlet saturation temperature of 30.5°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.

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 2750kgm−2s−1, heat fluxes from 20 to 64Wcm−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.

Local measurements and a new flow pattern based model for subcooled and saturated flow boiling heat transfer in multi-microchannel evaporators

A comprehensive experimental campaign has been conducted to measure the local heat transfer coefficients during flow boiling of refrigerants in multi-microchannel evaporators. Two additional refrigerants (R245fa and R236fa) were tested in two silicon evaporators at three inlet subcoolings and at three outlet saturation temperatures. The test section backside temperatures were measured by a fine-resolution infrared (IR) camera providing a two-dimensional thermal map, which was used by solving the three-dimensional inverse heat conduction problem to obtain the local heat transfer coefficients on a pixel-by-pixel basis. The experimental results revealed different trends along the flow direction. The decreasing trend (when existing) at the beginning of the channel was attributable to the single-phase thermal developing flow, then heat transfer increased from the onset of subcooled flow boiling up until the onset of saturated flow boiling, and afterwards it decreased again until entering the annular flow regime where it started to pick up and rose significantly. Combining our new data together with our recent experimental work of Huang et al. (2016), a new flow pattern based model has been proposed for local heat transfer prediction, starting from single-phase flow all the way through to annular flow. This new model also included a new local heat transfer method for the subcooled flow boiling region since no truly local subcooled heat transfer prediction method can be found in the literature for microchannels. 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 ±30%. It successfully tracks the experimental trends without any jumps in predictions when changing flow patterns.

Thermal Response of Multi-Microchannel Evaporators During Flow Boiling of Refrigerants Under Transient Heat Loads With Flow Visualization

Multi-microchannel evaporators with flow boiling, used for cooling high heat flux devices, usually experience transient heat loads in practical applications. These transient processes may cause failure of devices due to a thermal excursion or poor local cooling or dryout. However, experimental studies on such transient thermal behavior of multi-microchannel evaporators during flow boiling are few. Thus, 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 flow conditions. The effects on the base temperature behavior of the test section, heat flux magnitude, mass flux, inlet subcooling, outlet saturation temperature, and fluid were investigated. The transient base temperature response, monitored by an infrared (IR) camera, was recorded simultaneously with the flow regime acquired by a high-speed video camera. For cold startups, it was found that reducing the inlet orifice width, heat flux magnitude, inlet subcooling, and outlet saturation temperature but increasing the mass flux decreased the maximum base temperature. Meanwhile, the time required to initiate boiling increased with the inlet orifice width, mass flux, inlet subcooling, and outlet saturation temperature but decreased with the heat flux magnitude. For periodic variations in heat flux, the resulting base temperature was found to oscillate and then damp out along the flow direction. Furthermore, the effects of mass flux and heat flux pulsation period were insignificant.

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 10mm long and 10mm wide, having 67 parallel channels, each 100×100μm2, separated by a fin with a thickness of 50μm. 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 2750kgm−2s−1, heat fluxes from 6 to 50Wcm−2, inlet subcooling of 5.8K, and a nominal outlet saturation temperature of 35°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 high-speed 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%.

Intermittent dewetting and dryout of annular flows

Flow visualisation of saturated flow boiling of refrigerant R245fa in a silicon parallel multi-microchannel evaporator, at low mass flux and moderate uniform heat flux, has been carried out with a high-speed digital camera. Using the time-strip technique to post-process the recorded image sequences has revealed profound details regarding the intermittent dryout mechanism of annular flows. Features of these observed phenomena have been successfully correlated with the dynamics expected from a ruptured metastable liquid film under shear. These observations can act as a basis for the development of new mechanistic critical heat flux prediction models and the design of future high heat flux devices. This study also reveals details concerning the nature of the droplet entrainment-deposition process and nucleate boiling in very thin annular liquid films.

Fine-resolution two-phase flow heat transfer coefficient measurements of refrigerants in multi-microchannel evaporators

The time-averaged two-phase flow heat transfer coefficients were determined with a very fine resolution based on infra-red (IR) temperature measurements conducted over a 1cm2 heated area of a multi-microchannel evaporator. This experimental investigation included two low-pressure refrigerants, R245fa and R236fa, as well as one medium-pressure new refrigerant R1234ze(E) flowing in four different test sections. The number of microchannels and their cross-sectional areas were the same for all the tested micro-evaporators, respectively, 67 and 100×100μm2. Channel entrances with and without inlet micro-orifices were tested for establishing stable flow. Using 90×90 pixel grid (and then averaged width-wise), 90 local heat transfer coefficients were measured from inlet to outlet, yielding a U-shape of the heat transfer coefficient trend, where the descending branch of the curve corresponds to the coalescing elongated bubble flow regime (CB), while ascending one represents the increasing heat transfer coefficient in the annular flow regime (AF). The effects of channel mass flux, wall heat flux, orifice expansion ratio, and fluid properties on the wall heat transfer coefficient were identified for a selected number of data sets. In terms of predicted heat transfer coefficient values, in both the CB and AF flow regimes, the experimental results were found to be in a good agreement with respective existing flow pattern-based heat transfer prediction methods. Whereas, in order to better reflect the obtained experimental U-shaped trend of the heat transfer coefficient close to the CB–AF flow transition (which is a churn flow), the joined model of these two was modified by proposing a new vapor quality buffer for the width of the transition.

Two-phase flow operational maps for multi-microchannel evaporators

The current paper presents new operational maps for several different multi-microchannel evaporators, with and without any inlet restrictions (micro-orifices), for the two-phase flow of refrigerants R245fa, R236fa, and R1234ze(E). The test fluids flowed in 67 parallel channels, each having a cross-sectional area of 100×100μm2. In order to emulate the power dissipated by active components in a 3D CMOS CPU chip, two aluminium microheaters were sputtered onto the back-side of the test section providing a 0.5cm2 each. Without any inlet restrictions in the micro-evaporator, significant parallel channel flow instabilities, vapor back flow, and flow maldistribution led to high-amplitude and high-frequency temperature and pressure oscillations. Such undesired phenomena were then prevented by placing restrictions at the inlet of each channel. High-speed flow visualization distinguished eight different operating regimes of the two-phase flow depending on the tested operating conditions. Therefore, the preferred operating regimes can be easily traced. In particular, flashing two-phase flow without back flow appeared to be the best operating regime without any flow and temperature instabilities.

Flow pattern-based flow boiling heat transfer model for microchannels

A new flow pattern-based prediction method for heat transfer coefficients in microchannels was developed based on recent experimental results for several multi-microchannel evaporators in silicon and copper and for single-microchannel tubes in stainless steel. In the present paper, some updates to the three-zone flow boiling model for slug flow are presented, including further proof that the dryout thickness is well represented by setting it equal to the measured channel roughness for the silicon, copper and stainless steel test surfaces. Next, a non-circular channel version of the Cioncolini–Thome unified annular flow model for convective boiling is proposed. These two methods are joined together into a flow pattern-based method using a new heat flux-dependent flow pattern transition criterion between slug flow and annular flow. The method predicts the results quite accurately and also captures the trends in the heat transfer coefficients well.

On-Chip Two-Phase Cooling With Refrigerant 85 $\mu{\rm m}$-Wide Multi-Microchannel Evaporator Under Hot-Spot Conditions

Hot-spots are present in micro-electronics and are challenging to cool effectively. This paper presents highly nonuniform heat flux measurements obtained for a pseudo-CPU with 35 local heaters and temperature sensors cooled by a silicon multi-microchannel evaporator with 85 μm wide and 560 μm high channels separated by 46 μm wide fins. A low pressure dielectric refrigerant, R245fa, was used as evaporating test fluid. The base heat flux was varied from 6 to 160 W/cm2 and the junction temperature always remained below 65°C, while the fluid inlet saturation temperature was 30.5°C. On-chip two-phase cooling was found to very effectively cool the hot-spots without inducing flow instabilities. Building on analogous uniform heat flux tests made on the same test section, the effects of position, orientation size, and strength of the hot-spots were analyzed.