Search

Flow boiling heat transfer enhancement using porous coatings in microchannels has been experimentally investigated with HFE-7200. Results of the coated microchannel heat sink were compared to baseline results in a plain, micro-milled copper microchannel heat sink at similar operating conditions, namely inlet pressure of 1 bar, mass flux of 200 – 400 kg/m2 s and inlet subcooling of 10 K at wall heat fluxes between 24.5 kW/m2 to 206.6 kW/m2. Flow visualisation results and SEM surface analysis are presented. The coated surface was densely populated with well-defined cavities between 0.6 µm to 3.3 µm wide. The plain channels had fewer cavities for a given area and these were larger, i.e. up to 6 µm. Bubble generation frequency in the coated channels is significantly higher than in the plain channels due to the presence of more favourable nucleation sites on the coated surface. Flow pattern development occurred similarly in both heat sinks, namely bubbly to slug, churn and annular flow with increasing heat flux, with earlier flow pattern transitions in the coated heat sink. Enhancement in microchannel flow boiling heat transfer was shown to be influenced by mass velocity and may reach up to 44% at low heat fluxes, where the nucleate boiling mechanism is dominant. It diminishes with increasing heat flux, corresponding to nucleate boiling suppression following flow regime transition. Pressure drop increase posed by the coated heat sink is relatively small in terms of overall system power consumption but appears to be influenced by mass flux, pressure oscillations as well as channel rewetting behaviour.

Copyright © 2021 The Author(s). The pool boiling process is one of the most effective heat transfer modes capable of transferring large amounts of heat with small temperature difference between the heated surface and the fluid. In addition, fundamental knowledge of pool boiling processes is the starting point of flow boiling research and applications. It is therefore no surprise that it has been, and still is, the subject of extensive research globally for quite some time and a critical analysis is now required in order to move forward with enhanced surface designs. The current on-going research focuses on the understanding of boiling fundamentals including bubble generation, growth and bubble dynamics. In this context, fluid-surface interaction is critical. In the first part of this two-part paper we present the factors and parameters affecting the above, starting with the criteria for gas/vapour entrapment, nucleation site stability and the superheat required for heterogeneous nucleation. The models predicting the incipience superheat are critically described, classified into phase instability and superheated boundary-layer based models. This first part includes bubble growth and departure models, elucidating the effect of surface topology and wettability that can inform and facilitate the design of enhanced surfaces that are presented in Part II [10]. Three fluids of industrial interest, i.e. FC-72, HFE7100 and water were used through the discussion, as examples, to represent low and high surface tension fluids and help the understanding of surface-fluid interactions and relation to possible heat transfer enhancements. Engineering and Physical Sciences Research Council, UK (Grant Reference: EP/S019502/1).

Miniaturised electronics pose challenging thermal demands, not only at chip-level power dissipation but also at the complete system-level heat rejection, in modern electronic packages. Chip-level power densities are projected to be as high as 4.5 MW/m2 in computer systems by 2026.

Recent advancements in the electronics industry led to smaller and more powerful systems. Therefore, efficient heat dissipation for cooling of microelectronics systems, integrated circuit chips, power semiconductor devices such as IGBTs and laser diodes is required. In such systems, heat fluxes in the order of MW/m2 need to be removed from small spaces, while maintaining the temperature below a certain design limit (Karayiannis and Mahmoud in Appl Therm Eng 115:1372–1397, 2017 [1]). Flow boiling in microchannels is one of the most promising methods for achieving these high cooling demands because it can dissipate large heat fluxes over a small surface area by utilizing the latent heat of the coolant (Wang in Appl Therm Eng 110:369–381, 2017 [2]). Literature review indicates that there are still disagreements on the prevailing flow patterns, heat transfer rates, and pressure drop trends; see Mahmoud and Karayiannis (Encyclopedia of Two-Phase Heat Transfer and Flow IV, 2018, pp 233–301 [3]). The main objective of the present work was to investigate the flow boiling flow patterns, heat transfer coefficient, and pressure drop characteristics in a square metallic microchannel at different inlet subcooling, mass flux and heat flux conditions, using water as the working fluid.

The research community is currently focused on developing next generation technological solutions to utilise the energy available in domestic and industrial waste. The recoverable energy could be significant; for example, a recent study in the UK [1] identified 48 TWh/year of recoverable waste heat (17% of all industrial energy use). Organic Rankine Cycles (ORC) are best suited to harness this waste heat as they convert low and medium grade waste heat to power (

The effect of surface material on the flow boiling characteristics in a multi-microchannel evaporator is described in this paper. HFE-7100, a dielectric and eco-friendly working fluid, was tested at atmospheric pressure, inlet sub-cooling of 5 K, base heat flux up to 433.5 kW/m2 and mass flux range 50–250 kg/m2s. Two heat sinks made of copper and aluminium were fabricated having the same channel dimensions giving a hydraulic diameter of 0.46 mm with a base area of 500 mm2. The average roughness was measured and the values were 0.286 μm and 0.192 μm for the copper and aluminium, respectively. The effect of surface material was not significant at low heat flux. However, it became significant at moderate and high wall heat fluxes. The flow patterns for the two microchannels were similar and included bubble, slug, churn and annular flow. The experimental results showed that the aluminium surface achieved on the average difference of 12% higher heat transfer coefficients than those found in the copper microchannel. It also gave higher flow boiling pressure drop; the average difference was up-to approximately 30%. The different surface microstructures in the two examined heat sinks could explain the different heat transfer and pressure drop behaviour.

Copyright © 2021 The Author(s). Heat transfer enhancement by surface modification has been extensively studied in the last twenty years. However, there remains a large discrepancy among researchers on the performance of enhanced surfaces even for the same fluid and surface preparation technique. The reasons of this discrepancy are not understood and are not discussed in past papers, including paper reviews. Part II of this two-part paper aims to present a detailed assessment of pool boiling heat transfer enhancement, relating this to Part I [1], which presented a critical assessment of fundamental concepts of heterogeneous nucleation. Current challenges in evaluating the performance of enhanced surfaces is first discussed. The performance of smooth and roughened surfaces is then discussed and the effect of fluid type is explained. Pool boiling data of two fluids, namely water and FC-72, on two enhanced substrate materials, i.e. copper and silicon were digitized and assessed in order to elucidate the reason for the discrepancy in published works and present future recommendations for heat transfer enhancement. The heat transfer enhancement mechanisms adopted by researchers were presented and critically discussed and compared. The paper contributes to the understanding of the effect of fluid-surface combinations and suggest guidelines for researchers to consider when evaluating the performance of enhanced surfaces. This will help the research community and industry to conclude on the best surface structure and surface manufacturing technique matching particular fluid of interest. Engineering and Physical Sciences Research Council, UK (Grant Reference: EP/S019502/1).

Two-phase flow heat transfer was examined in a single 5 mm inner hydraulic diameter square channel in a vertical orientation. The channel uses a resistive coating to allow for transparent heating of the walls. Transparency of the heating enabled high speed visualization of the boiling phenomena at various heat and mass fluxes. The pressure is monitored at the inlet and outlet of the channel. Infra-red thermography is used to map the external wall temperature of the channel and the local heat transfer coefficient is estimated from the local wall temperature and the saturation temperature of the liquid. Ethanol, deionized water and a 5% v/v ethanol/water mixture were used as working fluids. Three mass fluxes (0.33, 0.66 and 1.00 kg/m 2 s) were tested as well as three heat fluxes (2.8, 4.2 and 6.1 kW/m 2 ). Experiments were conducted in a controlled temperature environment, where the surrounding air was kept at 40 °C. The addition of ethanol into water (5% v/v ethanol/water mixture) was found to enhance heat transfer resulting in higher heat transfer coefficients than for either of its pure components.

The main objective of this research was the design and test of a multi-microchannels heat sink for electronics cooling applications, which operates at system pressure near atmospheric and low mass flow rates. HFE-7100, a dielectric and eco-friendly coolant, was chosen as the working fluid. Twenty five rectangular microchannels, of width 0.7 mm and height 0.35 mm giving a hydraulic diameter of 0.46 mm, were fabricated from oxygen-free copper with a base area of 500 mm2. The channels in-between wall was 0.1 mm thick. Five mass fluxes ranging from 50 to 250 kg/m2 s were tested at fixed inlet sub-cooling near 5 K. The effect of heat flux, mass flux and vapour quality on the local heat transfer coefficient was investigated. Four flow patterns namely; bubbly, slug, churn and annular flow, were visualized using a high-speed camera mounted on a microscope. In this study, the maximum flow boiling heat transfer coefficient was 12.71 kW/m2 K at mass flux of 250 kg/m2 s. A comprehensive comparison with experimental results was conducted including flow pattern maps, heat transfer and pressure drop correlations. Some of the correlations, proposed for conventional channels and microchannels, showed good agreement with the present results.

Engineering & Physical Sciences Research Council (Grant EP/N011112/1 ).

Pressure changes caused by the growth of confined bubbles during flow boiling in mini-/microchannels lead to transient flow reversal in the presence of inlet (upstream) compressibility. A one-dimensional (1-D) model is presented to study the effect of inlet resistance on maximum flow reversal distance, local pressure fluctuations for different initial upstream compressible volumes, channel dimension, locations of nucleation site, heat flux, and initial channel velocity for water and FC-72 at atmospheric pressure and R134a at 800 kPa. The two upstream compressibility models considered are condensable vapor in a subcooled boiling region and trapped noncondensable gas.

The effect of channel aspect ratio on flow boiling characteristics (flow patterns, heat transfer and pressure drop) of HFE-7100 in copper multi-microchannel heat sinks was investigated experimentally. Three heat sinks with base area 500 mm2, channel hydraulic diameter 0.46 mm and channel aspect ratio (ratio of channel width to channel height) of 0.5, 1.0 and 2.0 were tested. The average surface roughness of the channel bottom surface was nearly the same in the three heat sinks and the measured values were 0.271, 0.286 and 0.304 µm. The local heat transfer rates were determined simultaneously with flow visualisation at mass flux ranging from 50 to 250 kg/m2s, wall heat flux from 9.6 to 191.6 kW/m2, system pressure of 1 bar and low inlet sub-cooling of 5 K. The results showed that, when the channel aspect ratio increased, the heat transfer coefficient increased, while the flow boiling pressure drop decreased. However, the heat transfer rate calculated using the heat sink base area was higher in the heat sink with the smallest channel aspect ratio, indicating an enhancement due to the largest surface area.

Numerical simulations were performed using Fluent 14.5 to investigate single phase flow and conjugate heat transfer in copper rectangular microchannels. Two different configurations were simulated: (1) single channel with hydraulic diameter of 0.561 mm and (2) multichannel configuration consisting of inlet and outlet manifolds and 25 channels with hydraulic diameter of 0.409 mm. In the single channel configuration, four numerical models were investigated namely, 2D thin-wall, 3D thin-wall (heated from the bottom), 3D thin-wall (three side heated) and 3D full conjugate models. In the multichannel configuration, only 3D full conjugate model was used. The simulation results of the single channel configuration were validated using experimental data of water as a test fluid while the results of the multichannel configuration were validated using experimental data of R134a refrigerant. In the multichannel configuration, flow distribution among the channels was also investigated. The 3D thin-wall model simulation was conducted at thermal boundary conditions similar to those assumed in the experimental data reduction (uniform heat flux) and showed excellent agreement with the experimental data. However, the results of the 3D full conjugate model demonstrated that there is a significant conjugate effect and the heat flux is not uniformly distributed along the channel resulting in significant deviation compared to the experimental data (more than 50%). Also, the results demonstrated that there is a significant difference between the 3D thin-wall and full conjugate models. The simulation of the multichannel configuration with an inlet manifold having gradual decrease in cross sectional area achieved very reasonable uniform flow distribution among the channels which will provide uniform heat transfer rates across the base of the microchannels.

An experimental study of flow boiling heat transfer and pressure drop was conducted using R245fa in stainless steel, brass and copper tubes of 1.1 mm internal diameter. Experimental conditions include: mass flux range 100–400 kg/m 2 s, heat flux range 10–60 kW/m 2 , pressure of 1.8 bar and exit vapour quality range 0–0.95. The tube surfaces were compared using scanning electron microscopy (SEM) and surface data acquired from confocal laser microscopy (CFLM), both showing differences between the surfaces. The heat transfer coefficient is similar in magnitude for all three materials but with a slight variation in trend. The heat transfer coefficient is seen to peak at high vapour qualities for stainless steel and brass, which is less evident with copper. The results were compared with past heat transfer correlations. These results showed better agreement with stainless steel compared to copper and brass. The pressure drop was shown to differ with surface characteristics, with the pressure drop for brass having a much steeper increase with heat flux. The pressure drop correlations tested did not show good agreement with the experimental results.

The experimental results of two-phase flow boiling pressure drop of R134a in vertical microdiameter stainless steel tubes are presented in this paper. The tests were conducted using four tubes: one tube with an inner diameter of 0.52 mm and 100 mm heated length and three tubes with an inner diameter of 1.1 mm and different heated lengths (150, 300, and 450 mm). Other experimental conditions include mass flux range of 200–500 kg/m2-s, system pressure range of 6–10 bar, inlet subcooling value of about 5 K, and heat flux range of 1–140 kW/m2. The results indicated that the total measured two-phase pressure drop increases with increasing mass flux, heat flux (exit quality) and decreasing system pressure and tube inner diameter. The test section heated length was found to have a significant effect on the measured pressure drop per metre length. The total measured two-phase pressure drop results were also compared with eighteen macro- and microscale models and correlations.

In this study, we survey the alternative approach of using plasma technology to produce CO x free hydrogen. In a corona discharge reactor operating at atmospheric pressure, electrons were energized by an electric field to ionize propane and induce chemical reactions. A range of each test parameter was covered, namely the effect of power input in the range of 4.5–105 W and discharge time of 1, 2, 3, 4, 5 and 8.18 min. A 19% hydrogen content by volume was achieved at a power input of 102 W and discharge time of 8.18 min. Copyright , Oxford University Press.

Accurate predictions of two-phase pressure drop in small to micro-diameter passages are necessary for the design of compact and ultra-compact heat exchangers, which find wide application in process and refrigeration industries and in the cooling of electronics. A semimechanistic model of boiling two-phase pressure drop in the confined bubble regime is formulated, following the three-zone approach for heat transfer. The total pressure drop is calculated by time-averaging the pressure drops for single-phase liquid, elongated bubble with a thin liquid film, and single-phase vapor. The model results were compared with experimental data collected for a wide range of tube diameters (4.26, 2.88, 2.02, 1.1, and 0.52 mm) for R134a at pressures of 6–12 bar. In this model's present form, its predictions are close to those of the homogeneous flow model but it provides a platform for further development.

Boiling heat transfer in small diameter tubes has been experimentally investigated using R134a as the working fluid. The heat transfer experiments were conducted with two stainless steel tubes of internal diameter 4.26 and 2.01 mm respectively. Other parameters were varied in the range: mass flux 100–500 kg/m2 s; pressure 8–12 bar; quality up to 0.9; heat flux 13–150 kW/m2. It was found that the nucleate boiling is dominant when the vapour quality is less than about 40–50% for the 4.26 mm tube and 20–30% for the 2.01 mm tube. Above these quality values, heat transfer coefficients decrease with vapour quality. Furthermore, this decrease occurs for the entire quality range at high heat flux values. Flow visualization experiments were carried out using the same experimental facility with Pyrex glass tubes. A flow pattern map was obtained at a system pressure of 10 bar and tube diameter of 4.26 mm.

An experimental study of low level turbulence natural convection in an air filled vertical square cavity was conducted. The dimensions of cavity were 0.75 m × 0.75 m × 1.5 m giving two-dimensional flow. The hot and cold walls of the cavity were isothermal at 50 and 10°C, respectively, giving a Rayleigh number of 1.58 × 109. The temperature and velocity distribution was systematically measured at different locations in the cavity, and was nearly anti-symmetrical. An experimentally obtained contour plot of the thermal field and a vector plot of the air flow in the cavity are reported for low turbulence natural convection in such cavities for the first time. The wall shear stress and the local and average Nusselt numbers are also presented. The Nusselt number compares well with previous results; the agreement on the velocity and temperature profiles at mid-height near the vertical walls is fair. Differences were found at mid-width and in the rate of velocity and temperature changes near the walls. The experiments were conducted with high accuracy. Therefore, the results can form experimental benchmark data and will be useful for CFD code validation.

The results of an experimental study of electrohydrodynamic (EHD) enhancement of pool boiling are presented in this paper. A high intensity dc electric field was applied to R123 and R11 boiling on the shell side of a smooth five-tube shell and tube heat exchanger. The tubes were electrically heated and were arranged in three rows. The electrode system carrying the high voltage comprised fourteen mild steel rods placed in between and around the tubes. Experiments were performed at heat flux values of 20–5 kW m −2 and applied voltages of 0–25 kV. The enhancement ratio for R123 increased linearly with an applied voltage of up to 15 kV and was strongly dependent on heat flux. Increasing the voltage beyond 20 had no effect on the enhancement ratio at heat flux values greater than 10 kW m −2 . At lower heat flux, some further increase in the enhancement ratio was obtained when increasing the voltage from 20 to 25 kV. The enhancement with R11 was marginal, indicating the importance of fluid properties to electrohydrodynamic enhancement. The effect of pressure on the enhancement was also examined.

Experimental results for etectrohydrodynamic (EHD) effects on nucleate boiling at passively enhanced surfaces, such as Thermoexcel-HE and Gewa-T, are reported. The experiments were carried out using a single-tube test rig by applying a 0- to 30- kV dc potential to a dielectric liquid (Refrigerant-114). The EHD effect on the enhanced surfaces is compared with results of earlier experiments on smooth and Thermoexcel-C tubes and discussed. The results indicate that the boiling hysteresis at these passively enhanced surfaces can be eliminated and the nucleate boiling can be further enhanced and controlled by applying an electric field. An explanation of bubble initiation at low superheat by an electric field is offered in terms of the effect of the field on bubble critical radius.

The electrohydrodynamic (EHD) enhancement mechanism is first outlined in this paper. A comprehensive review of past work on EHD single and two-phase heat transfer, as well as past work in the related area of EHD-induced flow, is then presented. Correlation attempts are also reviewed. Recent experimental results for EHD boiling and condensation in single- and multi-tube heat exchangers are discussed. A description of the possible practical EHD electrode systems for applications in power production cycles and regrigeration is also presented. The research work needed to clarify outstanding questions in EHD and encourage its use in practical systems is discussed.

Copyright @ 2011 Elsevier Two-phase flow boiling in microchannels is one of the most promising cooling technologies for coping with high heat fluxes produced by the next generation of central processor units (CPUs). If flow boiling is to be used as a thermal management method for high heat flux electronics it is necessary to understand the behaviour of a non-uniform heat distribution, which is typically the case observed in a real operating CPU. The work presented is an experimental study of two-phase boiling in a multi-channel silicon heat sink with non-uniform heating, using water as the cooling liquid. Thin nickel film sensors, integrated on the back side of the heat sinks were used in order to gain insight related to temperature fluctuations caused by two-phase flow instabilities under non-uniform heating. The effect of various hotspot locations on the temperature profile and pressure drop has been investigated. It was observed that boiling inside microchannels with axially non-uniform heating leads to high temperature non-uniformity in the transverse direction. This research was supported by the UK Engineering and Physical Sciences Research Council through grant EP/D500109/1.

Convective heat transfer was investigated numerically for rectangular enclosures both undivided and divided in two zones by a vertical partition, and having opposite isothermal walls at different temperatures. The aspect ratio was varied from 0.1 to 16 and the Rayleigh number from 3.5 ∗ 10 3 to ∗ 10 7 (non-partitioned enclosures) and from 1.0 ∗ 10 5 to 1.6 ∗ 10 8 (partitioned enclosures). The thickness and conductivity of the partition were varied. The end wall thermal boundary conditions were adiabatic or LTP (Linear Temperature Profile). The continuity, momentum and energy equations for a 2-D laminar steady flow were solved under the Boussinesq approximation by using a finite-difference method and the SIMPLEC pressure-velocity coupling algorithm. Grid-independent results indicate that the reduction in the Nusselt number caused by a thin central partition can be predicted within a few per cent (in the range investigated) by assuming the partition to be isothermal, i.e. infinitely conducting. The finite conductivity of the partition causes a temperature distribution along its length, resulting in an increase in Nu which depends on Rayleigh number, aspect ratio and end wall thermal boundary conditions.

The effect of symmetric partitions protruding centrally from the end walls of a rectangular vertical enclosure on heat transfer rates is investigated numerically. The enclosure has opposite isothermal walls at different temperatures. The Rayleigh number is varied from 10 4 to 10 7 and the aspect ratio from 0.5 to 10. The thickness of the partitions is fixed and equal to one tenth of the width of the enclosure. Their non-dimensional length ( L / H ) is varied from 0 (non-partitioned enclosure) to 0.5 (two separate enclosures). The effect of different thermal boundary conditions at the end walls and at the partitions is included in the investigation.

Flow boiling heat transfer experimental results, obtained in two stainless steel tubes of internal diameter 4.26 mm and 2.01 mm using R134a as the working fluid, indicate that the local heat transfer coefficient increases with heat flux and is independent of vapour quality when this is less than about 40% to 50% for the 4.26 mm tube and 20% to 30% for the 2.01 mm tube, conventionally interpreted as nucleate boiling. Above these quality values, the separate graphs merge into a single line for heat transfer coefficient decreasing with increasing vapour quality. The data in the apparently-nucleate boiling condition are compared with a recent state-of-the-art three-zone evaporation model for the confined bubble flow regime without a nucleate boiling contribution. The model predicts the experimental data reasonably well but does not predict correctly the trends for changing pressure and diameter. Some suggestions are made for improving the model. The comparisons made in this paper support the statements by the developers of the model and others that the application of conventional macro flow boiling correlations to micro tube flow boiling heat transfer may not necessarily have a sound physical basis.

An interferometric technique was used to study coupled convective heat transfer for an inclined solar collector. The collector was simulated by an inclined, closed rectangular cavity which was heated isothermally at the bottom plate and cooled by natural convection at the top plate. Both plates had dimensions 457.2 mm wide and 424.2 mm long. Infinite interferograms, for temperature field visualization, and finite interferograms for analysis were obtained. The parameters studied were Rayleigh number, aspect ratio, and angle of inclination. The Rayleigh number was varied from subcritical to 6 × 105 and the aspect ratio was changed from 6.68 to 33.4. The angle of inclination was 15° and 60° to the horizontal. The temperature on the cold plate was found to increase with distance from the lower boundary. Temperature reversal occurs in the cavity and first appears in the corner regions. The temperature gradients were obtained and thus the local heat transfer coefficients were calculated indicating a strong dependence from side boundaries. In comparison with reports where both surfaces were isothermal a significant difference in the dependence of average Nusselt number on Rayleigh number was found. At high Rayleigh numbers the average Nusselt number is less dependent on Rayleigh number for the cavity with a non-isothermal top surface than for the isothermal top surface.

Natural convection inside a rectangular cavity with different temperature boundary conditions on the cold top plate was studied using a Mach-Zehnder interferometer for {theta} = 15, 45, and 60 deg to the horizontal. At {theta} = 60 deg coupling with external forced convection and non-coupled heat transfer from a cavity with an isothermal top plate was studied. In all experiments the bottom hot plate was isothermal. The Rayleigh number Ra was varied from subcritical to 6 {times} 10{sup 5} and the cavity aspect ratio AR{sub x}, from 6.68 to 33.4. The Reynolds number of the external forced flow Redn was constant and approximately equal to 5.8 {times} 10{sup 4}. It was found that for Ra {approx lt} 3 {times} 10{sup 4} the differing thermal boundary conditions at the top plate did not affect the local or average heat transfer rates from the cavity. For Ra {approx gt} 3 {times} 10{sup 4} coupling at the top plate compared to the non-coupled case resulted not only in a reduction in the variation of the local heat transfer rates at the cold plate, but also in a significant reduction in the variation of the average transfer rates from hot and cold plates onmore » the cavity. Forced convection at the top plate as compared to natural convection resulted only in a small reduction in the heat transfer coefficient at the cold plate. Correlation equations for coupled and noncoupled average heat transfer rates are presented.« less