Heat Flux Range Research Articles

Embedded cooling with 3D manifold for vehicle power electronics application: Single-phase thermal-fluid performance

Single-phase thermal-fluidic performance of an embedded silicon microchannel cold-plate (25 parallel channels: 75 μm × 150 μm) with a 3-D liquid distribution manifold (6 inlets: 700 μm × 150 μm) and vapor extraction conduits, is investigated using water as working fluid. A 3D manifold is fabricated from silicon and bonded to a silicon microchannel substrate to form a monolithic microcooler (μ-cooler). A metal serpentine bridge (52 mm2 of footprint) and multiple resistance temperature detectors (RTDs) are used for electrical Joule-heating and thermometry, respectively. The experimental results for maximum and average temperatures of the chip, pressure drop, thermal resistance (as low as 0.68 K/W), average heat transfer coefficient (∼30,000–50,000 W/m2 K) for flow rates of 0.03, 0.06 and 0.1 l/min and heat fluxes of 60, 100 and 250 W/cm2 are reported. The embedded microchannel-3D manifold μ-cooler device is capable of removing 250 W/cm2 at a maximum temperature of 90 °C with less than 3 kPa pressure drop for a flow rate of 0.1 l/min. The results from conjugate thermal-fluidic numerical simulations agree well with the experimental data over the wide range of heat fluxes and flow conditions. The numerical simulation results also hint at the possibility of removing up to ∼850 W/cm2 using single-phase water at a maximum temperature of 166 °C at the same pressure drop and flow rate. This offers a very attractive strategy/option for cooling of high heat flux power electronics using single-phase water.

Electrical response of thermoelectric generator to geometry variation under transient thermal boundary condition

A three-dimensional numerical model is applied in this study to illustrate the electrical response of a thermoelectric generator (TEG) during transient heat flux at the hot side. In this work, various types of thermal boundary conditions are considered to evaluate the performance of the TEG. Thus, a TEG under pulsed heat flux is studied numerically, and the numerical model is verified by experimental results. With the consideration of a defined reference geometry, different heat flux frequencies are applied in order to evaluate the corresponding electrical output by the TEG. In addition, variation of the module performance for various TEG leg lengths and its cross-sectional area are studied over a wide range of heat fluxes. The results indicate that the open circuit voltage in the experiment is in a good agreement with the open circuit voltage in the simulation results. The results show that the range of power oscillation reduces at higher frequency of the applied heat flux. Furthermore, the variability of the output power increases as the thermoelectric element length increases and the area of the element reduces.

Multi-nozzle Closed Loop Spray Cooling Systems in Electronics Cooling

This study presents two refrigerant closed loop spray cooling systems for two specific electronic cooling applications. A small system using a 3 × 2 nozzle array is developed to investigate the spray cooling performance on a concentrated heat source (2 × 1 cm2) with a high heat flux. A big system uses a 9 × 6 nozzle array to cool a 6U card area (23.3 × 16 cm2) with a moderate uniform heat flux. The experimented flow rate in the small system is maintained at 7.8∼8.1 g/s corresponding to a spray mass flux of 3.9∼4.05 g/cm2⋅s on the concentrated heat source, and the studied flow rates in the big system are monitored at 113 ± 4 g/s corresponding to a spray mass flux of 0.30 g/cm2⋅s on the 6U card surface area. The results show that the small system with a much higher spray mass flux can perform a spray cooling curve with a wide heat flux range. The big system with much less spray mass flux has performed a slightly better heat transfer coefficient due to its much better evaporation efficiency at the same degree of surface superheat.

Experimental and theoretical analysis on the safety and efficiency of an ultra-supercritical pulverized coal-fired boiler with low mass flux vertical water wall

The heat transfer safety of low mass flux vertical water wall suitable for a high efficiency and wide-load ultra-supercritical coal-fired boiler is investigated experimentally. The pressure drop characteristics of supercritical water are discussed. The net thermal efficiency of an ultra-supercritical pulverized coal-fired power plant is analyzed theoretically by considering the power consumption of the feed water pump. The experimental results demonstrate the test water wall tube exhibits good heat transfer and low pressure drop characteristics at low mass flux and in the normal heat flux range. However, the self-compensation characteristic between parallel tubes can be destroyed at high test heat flux when the test pressure is near the critical value. In this case, the metal temperature and pressure drop of the water wall tube increase fast and abnormally. This phenomenon indicates the distributions of in-tube mass flux and furnace heat flux no longer match, which threatens the heat transfer safety of the water wall. Corresponding analysis demonstrates that on the premise of ensuring the heat transfer safety of the water wall, adopting the low mass flux technology can improve net thermal efficiency of the power plant.

Experimental investigation of condensation heat transfer and pressure drop of R-134a flowing inside dimpled tubes with different dimpled depths

An experimental investigation is conducted to determine the effect of dimpled depth on the condensation heat transfer coefficient and pressure drop of R-134a flowing inside dimpled tubes. The test condenser is a double tube heat exchanger where the refrigerant flows inside and water flows in the annulus. The inner tube is a 1500 mm long and 8.1 mm inside diameter. The experiments are carried out for one smooth tube and three dimpled tubes having dimpled depth of 0.5, 0.75, and 1.0 mm. For each test tube, several test runs are performed over mass flux range of 300–500 kg/m2s, heat flux range of 10–20 kW/m2, and condensing temperature range of 40–50 °C. The experimental results reveal that the dimpled tube presents the significant heat transfer enhancement and pressure drop penalty. The tube with the highest dimpled depth yields the highest heat transfer enhancement and pressure drop penalty up to 83% and 892% higher than those of the smooth tube, respectively. Additionally, the overall performance of dimpled tube is evaluated in term of efficiency index. The new correlations including the effect of dimpled depth, dimpled pitch, and helical pitch on the Nusselt number and friction factor of R-134a in dimpled tube are developed.

Boiling heat transfer and flow characteristic of R32 inside a horizontal small-diameter microfin tube

This study experimentally investigated the flow boiling heat transfer and pressure drop characteristics of R32 inside a horizontal small-diameter microfin tube with an equivalent diameter of 3.48 mm. The boiling heat transfer coefficient and pressure drop were measured in a mass velocity range of 50–400 kgm−2 s−1, heat flux range of 5–20 kWm−2, and saturation temperature of 15 °C. The flow pattern at the outlet of the test microfin tube was observed and classified as wavy-slug flow and annular flow regime. The effects of quality, mass velocity, heat flux, and flow pattern on the flow boiling characteristics were clarified. The heat transfer mechanism was characterized by forced convection, nucleate boiling, and evaporation heat transfer through the thin liquid film inside the grooves. No difference in the frictional pressure drop on adiabatic and boiling flows was observed. The measured heat transfer and frictional pressure drop were compared with those in previous correlations.

The development of macrolayer thickness of water in the pool boiling regime

ABSTRACT The main purpose of this study is to analyse the phenomenon of pool boiling in water over a horizontally placed copper tube heater of 28 mm diameter. The experiment has been carried out to observe the bubble growth and departure characteristics for the heat flux range up to 40,000 W/m2. The entire process is being recorded by a digital camera at different time intervals. The measured parameters have been used to determine the initial layer thickness, macro layer thickness and critical heat flux and validated with models proposed in the literature.

An experimental study and comparitive validation of macrolayer thickness in nucleate pool boiling for horizontal copper tube heater

Nucleate Pool boiling is characterised by the progression of several nucleation sites (bubble formation), which surge from distinct points on a surface, whose temperature is slightly above the liquids. To investigate and evaluate this, a pool boiling setup was fabricated with a horizontal copper tube heater of 28mm diameter by imposing cartridge heater. An analytical expression, suggested by literature helps to determine the thickness of macrolayer based on bubble diameter was used. The macrolayer thickness for water was found by measuring the bubble diameter by using Photographic and CAD method. Experiment was carried out in a stainless steel container insulated with Teflon cover to observe the bubble growth and bubble departure characteristics for the heat flux range of 1000-42,000 W/m2. The bubble diameter were measured and the measured parameters were been used to determine the initial layer thickness, macro layer thickness and critical heat flux and validated through various models. The observed and calculated values are in good agreement as reported in various literature.

An experimental study of the effect of nanoparticle additives to the refrigerant r141b on the pool boiling process

The results of the experimental study of the internal characteristics of the pool boiling process of the refrigerant R141b, solution R141b/surfactant Span-80 and nanofluid R141b/Span-80/ TiO 2 nanoparticles on the surfaces of stainless steel and teflon have been presented. The measurement of the vapor bubble departure diameter, the vapor bubble departure frequency and the nucleation site density has been performed at atmospheric pressure and in the range of heat fluxes from 3.0 to 7.5 kW·m -2 . The study showed that the vapor bubble departure diameter in nanofluid boiling on the stainless steel surface is 0.7 mm and on the teflon surface – 0.45 mm. Besides, the additives of nanoparticles to the solution of R141b/Span-80 lead to a decrease in the vapor bubble departure diameter in boiling on the teflone surfaces. The opposite effect was detected in boiling on the stainless steel surface. It is shown that the additives of TiO 2 nanoparticles to the solution R141b/Span-80 lead to a decrease in the number of nucleation sites by 2–8 times. This effect depends on the heat flux and type of heaters surface. It was found that the rise of the heat flux leads to an increase in the difference between the magnitudes of nucleation site density for the teflon and stainless steel surfaces in boiling of R141b and R141b/Span-80. The number of nucleation sites on the teflon surface is 2 times lower compared with boiling on the stainless steel surface at a heat flux of 7.5 kW·m -2 . The type of surfaces does not affect the number of nucleation sites and vapor bubble departure frequency in nanofluid boiling in the entire investigated range of heat fluxes. Based on the results of the study, it was found that the vapor bubble departure frequency in boiling of R141b and solution R141b/Span-80 on the teflon surface is 1.5–2 times lower compared with boiling on the stainless steel surface. The obtained experimental data can be used in predicting the heat transfer coefficient in boiling of the solution of R141b/Span-80 and nanofluid R141b/Span-80/TiO 2 .

Thermo-structural design of the European DEMO water-cooled blanket with a multiscale-multiphysics framework

The Water-Cooled Lithium-Lead (WCLL) blanket concept is one of four possible embodiments under consideration for the European Demonstration Power Plant DEMO, beyond ITER. It is based on a water-cooled first wall and blanket structure with the RAFMS (Reduced Activation Ferritic/Martensitic Steel, Eurofer 97) as structural material and PbLi as neutron multiplier and tritium breeder. We first develop a multiphysics modeling framework in order to optimize the design and achieve long blanket lifetime, maintainability, and high reliability. The approach is demonstrated by coupling Computational Fluid Dynamics (CFD), heat transfer and solid mechanics. The multiphysics framework is then utilized to provide the loading conditions for a multiscale design approach particularly focused on modeling the effects of both steady-state and operational transients on the structural integrity of the First Wall (FW). The methodology builds on the ITER Structural Design Criteria for In-Vessel Components (ISDC-IC), then extending the criteria to progressively incorporate continuum plasticity models and microstructure-based representation of deformation and fracture. We finally introduce advanced fracture mechanics concepts based on a Materials-Specific Failure Assessment Diagram (MS-FAD), in which precise calculations of the J-integral of an elasto-plastic material is required. This is especially important for the water-cooled design because of the limited fracture toughness of Eurofer 97 below 300 °C. The methodology has been applied to screen the FW/Blanket performance of a range of steady state heat flux assumed to be applied during the plasma pulse flat top, and up to 0.8 MW/m2. It has become clear that the radiation steady state load will be lower than the values screened here, while the local heat flux due to particle loads will lead to global higher values than the values investigated here. The methodology will be further fine-tuned to specific loading conditions in order to assess actual performance limits of the DEMO WCLL BB.

Experimental investigation of geyser boiling in a two-phase closed loop thermosyphon with high filling ratios

The geyser boiling instability in a two-phase closed loop thermosyphon (TPCLT) is experimentally investigated through flow visualization and simultaneous measurement of pressure and temperature fluctuations. Wide ranges of filling ratios of R134a fluid from 90% to 53%, and heat flux from 20 W cm−2 to 220 W cm−2 are examined. The Power Spectrum Density (PSD) method is applied to analyze the periodicity of geyser boiling, and the parameter Standard Deviation (SD) is used to characterize the oscillating amplitude. The effects of heat flux and filling ratio on the geyser boiling occurrence and the oscillation characteristics are discussed in detail. The results show that, the flow regimes experience the consecutive variation of single-phase flow, bubbly flow, churn flow, bubbly flow, and single-phase flow within each geyser boiling cycle, leading to the fluctuation of flow and heat transfer characteristics. The geyser boiling is more liable to occur in the conditions of higher filling ratio and moderate heat flux. The initiating heat flux for the onset of geyser boiling decreases with the increase of filling ratio, but the range of heat flux for the geyser boiling occurrence is narrow under the high filling ratio conditions. The frequency of geyser boiling increases with the increase of heat flux for a certain filling ratio. With the increase of filling ratio, the oscillating frequency firstly decreases and then increases. The minimum oscillating frequency occurs at the combination of filling ratio of 76% and heat flux of 90 W cm−2 under the experimental conditions in this work. Both the fluctuation amplitude of pressure and temperature increase with the increase of heat flux, while decrease with the increase of filling ratio. Compared to R134a under the same filling ratio of 76%, the water has higher heat flux for geyser boiling occurrence, smaller oscillating amplitude of pressure, and lower oscillating frequency due to the difference in thermophysical properties.

Pool boiling heat transfer enhancement with segmented finned microchannels structured surface

An experimental investigation has been carried out to investigate pool boiling heat transfer characteristics of segmented finned (SF) microchannels structured surface and compare its performance with that of uniform cross-section (UCS) microchannels structured surface and plane surface. All three surfaces have been fabricated on individual copper block with a foot print area of 10 × 10 mm2. Pool boiling experiments have been performed with these surfaces in atmospheric pressure condition using deionized water. Experiments have been performed for applied effective heat flux range of 0–200 W/cm2. Both the structured surfaces show better heat transfer performance compared to plane surface. It has been observed that SF structured surface shows a heat transfer improvement up to a factor of 3 times the heat transfer coefficient in plane surface whereas uniform UCS structured surface shows the improvement by a factor of 2 times the heat transfer coefficient in plane surface. Thus segmented finned microchannels structured configuration shows better heat transfer performance compared to other two surfaces. The reason behind the heat transfer improvement in SF configuration might be due to more number of active nucleation sites, better rewetting phenomenon and favorable bubble growth and release mechanism.

Comparison of normal and distributed jet array impingement boiling of HFE-7000 on smooth and pin-fin surfaces

The confined jet impingement boiling experiments are conducted with HFE-7000 as coolant to compare bubble dynamics and heat transfer characteristics of normal and distributed jet arrays on smooth surfaces and pin-fin surfaces, respectively. It is found that in single-phase regime, the two jet arrays have almost the same heat transfer coefficient (HTC) because of the negligible crossflow effect due to small jet velocities and short flow path. This is especially true on pin-fin surfaces owing to its further attenuation effect on crossflow. For each test section, the higher the flow rate is, the larger the single-phase coefficient (HTC) and critical heat flux (CHF) are, whereas the flow rate has negligible effect on HTC in fully developed nucleate boiling regimes due to the dominant role of nucleate boiling heat transfer. On smooth surfaces, nucleate boiling starts at the center of two jet rows for both jet arrays where the heat transfer is the weakest before boiling. Nevertheless, the wall superheat at the onset of nucleate boiling (ONB) for the distributed jet array is slightly lower under the same flow rate both on the smooth and pin-fin surfaces. For normal jet array, quantities of bubbles generated at high heat flux deteriorate the flow rate distribution and consequently results in the very different boiling characteristics along the streamwise direction. For distributed jet arrays, bubble characteristics are the same for each unit because of the absence of crossflow effect, and therefore its two-phase HTC and CHF are higher. This feature of distributed jet array makes it practical to large area cooling applications. Pin-fin surfaces can greatly decrease ONB, enhance HTC during whole heat transfer regimes and increase CHF for both jet arrays. Meanwhile, the boiling curves of the two jet arrays are overlapped in a wider range of two-phase regime due to the pin-fin enhancement on nucleate boiling and the attenuation of crossflow, while distributed jet arrays have much higher CHFs. For distributed jet arrays on pin-fin surfaces, boiling instability occurs at a certain range of heat flux owing to the periodical accumulation and extraction of large individual bubbles under effusion holes. The boiling instability, however, has no detrimental effect because it only leads to a slight drop in HTC and does not cause earlier occurrence of CHF.

Exploration of thermal counterflow in He II using particle tracking velocimetry

Visualization of thermal counterflow in He II using PIV (particle image velocimetry) and PTV (particle tracking velocimetry) is difficult because tracer particle motion can be influenced by both the normal fluid and superfluid components of He II as well as the quantized vortex tangle. For instance, an early PTV experiment observed particles moving at the normal fluid velocity $v_n$, while a PIV experiment observed particles moving at $v_n/2$, though the range of heat flux applied in these experiments differed by an order of magnitude. To resolve this apparent discrepancy and explore statistics of particle motion in thermal counterflow, we have applied PTV to a wide range of heat flux at several fluid temperatures. We introduce a scheme for analyzing the velocity of particles presumably moving with the normal fluid separately from those presumably influenced by the vortex tangle. Our results show two distinct peaks in the streamwise particle velocity PDF (probability density function) for lower heat flux, one centered at the normal fluid velocity $v_n$ ("G2") and one near $v_n/2$ ("G1"). For higher heat flux there is a single peak centered near $v_n/2$ ("G3"). Using our separation scheme we show there is no size difference between particles contributing to G1 and G2. We also show that non-classical features of the transverse velocity PDF arise entirely from G1, while the corresponding PDF for G2 exhibits classical Gaussian form. G2 transverse velocity fluctuation, backed up by second sound attenuation in decaying counterflow, suggests large scale turbulence in the normal fluid is absent from the two peak region. We offer a brief discussion of physical mechanisms that may be responsible for our observations, revealing that G1 velocity fluctuations may be linked to fluctuations of vortex line velocity, and suggest numerical simulations that may reveal underlying physics in detail.

Two-Phase Flow In Mini-Scale Complex Geometry

Heat-transfer coefficients are reported for one surface, a plain surface, with 50 mm square base area. Parallel channel test piece has one mm by one mm, 25 channelsThe data were produced while boiling R113 at atmospheric pressure. For this surface, the mass flux range was 200 – 600 kg/m2s and the heat flux range was 5 - 80 kW/m2. The results obtained have been compared with standard correlations for tube bundles. The measured heat-transfer coefficients for the parallel micro-channel surface are slightly bigger for any plate channel surface. It is dependent on heat flux and reasonably independent of mass flux and vapor quality. Thus, heat transfer is probably dominated by nucleate boiling. The parallel channel heat transfer coefficients were typically bigger than other plate -channel values.

Single-phase and two-phase Flow into In-Line pin-fin micro-channel heat Sink

Heat-transfer coefficients are reported for one surface, a pin-fin surface with 50 mm square base area. The In Line pin-fin surface comprised of 1 mm square pin fins that were 1 mm high and located on a 2 mm square pitch array it that covering the base. The channel was 
 1 mm high and had a glass top plate. The data were produced while boiling R113 at atmospheric pressure. For this surface, the mass flux range was 50 - 250 kW/m2s and the heat flux range was 5 - 140 kW/m2. The results obtained have been compared with standard correlations for tube bundles. The measured heat-transfer coefficients for the pin-fin surface are slightly higher any surface. It is dependent on heat flux and reasonably independent of mass flux and vapor quality. Thus, heat transfer is probably dominated by nucleate boiling and is increased by the pin fins due to increasing in area and heat-transfer coefficient, the pin-fin pressure drops were typically larger than other values.

An experimental study of vapor bubbles dynamics at water and ethanol pool boiling at low and high heat fluxes

In this paper the results of experimental study of vapor bubbles dynamics at pool boiling of various liquids in a wide range of heat fluxes up to q/qCHF ∼ 0.9 are presented. The experiments were performed at boiling of saturated water and ethanol at atmospheric pressure with the use of high-speed experimental techniques including video macro-visualization and IR thermography from the bottom side of a transparent heated sample. As a result, new data on the growth rate of vapor bubbles and dry spots in their base, evolution of the liquid microlayer region and unsteady temperature field of a thin film heater surface were obtained, and analysis of patterns of process at low heat fluxes was carried out. The usage of high-speed experimental techniques also allowed in this study to investigate the evolution of vapor bubbles with formation of vapor patterns and large agglomerates, to study dry spots dynamics, to estimate void fraction close to heating surface at fully developed nucleate boiling regime up to the critical heat flux depending on liquid properties. Obtained experimental information can be further used to construct more accurate physical models for the theoretical description of microcharacteristics, heat transfer and boiling trigger mechanisms at nucleate boiling of liquids with different physical properties.

Evaporation Heat Transfer and Pressure Drop Characteristics of R32 Inside a Multiport Tube with Microfins

This study investigated the evaporation heat transfer and pressure drop characteristics of R32 in a horizontal multiport tube consisting of rectangular minichannels with straight microfins. The heat transfer coefficient and pressure drop were measured for a mass velocity range of 50–400[Formula: see text]kgm[Formula: see text]s[Formula: see text] and heat flux range of 5–20[Formula: see text]kWm[Formula: see text] at a saturation temperature of 15[Formula: see text]C. The frictional pressure drop during an adiabatic two-phase flow was also measured for a mass velocity range of 50–400[Formula: see text]kgm[Formula: see text]s[Formula: see text] and quality range of 0.1–0.9 at the same saturation temperature. The heat transfer coefficient increased with an increasing quality owing to the increase in forced convection. The dryout inception quality increased with the increase in mass velocity. The effects of heat flux on the heat transfer coefficient were small, except in a high-quality region. The heat transfer coefficient in a multiport tube with microfins was higher than that in a multiport tube without microfins in a high-quality region at a mass velocity of 200[Formula: see text]kgm[Formula: see text]s[Formula: see text] and in a low-quality region at a mass velocity of 400[Formula: see text]kgm[Formula: see text]s[Formula: see text]. The effects of mass velocity and microfins on the frictional pressure drop were clarified. It is suspected that the effects of a microfin on the frictional pressured drop can be considered using the hydraulic diameter. The frictional pressure drop was shown to be in good agreement with previous correlations.

Numerical investigation of jet impingement cooling of a low thermal conductivity plate by supercritical pressure carbon dioxide

Jet impingement cooling is widely used due to its high heat transfer rates, especially within the stagnation region. Jet impingement cooling by supercritical pressure fluids has even higher heat transfer coefficients with the proper working conditions than regular fluids with no burnout. This paper presents a numerical investigation of supercritical pressure carbon dioxide jet impingement cooling of a low thermal conductivity plate. Predictions of 11 RANS turbulence models were first validated against the published experimental data done by our research group. The effects of the inlet temperatures and pressures on the heat transfer were studied numerically for a wide range of heat fluxes with the flow phenomena observed in the experiment further explained by the numerical results. The SST k-ω and Transition SST models gave more accurate predictions of the average heat transfer coefficient with differences of less than 15% for the validated case. The maximum average heat transfer coefficient occurred when the inlet temperature was lower than and the surface temperature was slightly higher than the pseudocritical temperature. The maximum average heat transfer coefficient increased with increasing inlet temperature when the inlet temperature was lower than the pseudocritical temperature and decreased with increasing pressure.

Experimental observation of the maximum bubble diameter in non-stationary temperature field of subcooled boiling water flow

This paper presents results of the experimental study of the subcooled boiling flow on the surface of the vertical cylindrical steel heater. The initial stages of formation of the fully developed nucleate boiling were investigated over the unsteady stepwise heat flux range from 0.56 to 1.76 MW/m2. The field temperature of heated liquid layers at the moment of the bubble departure was described by numerical modeling under nonstationary conditions. A high-speed digital video camera was used to capture the bubble dynamics. The effects of heat release conditions and inlet water temperature on the bubble maximum diameter and nucleation frequency were studied. We found that the increased heat flux results in reduction of the maximum diameter at non-stationary heat release. Calculations on the basis of prediction models showed that the non-stationary nucleate boiling data cannot be generalized using the models developed by Prodanovic et al. (2002) and Song (2016) with the coefficients obtained for the stationary boiling. However, the heat balance approach to calculation of the bubble maximum diameter proved to be promising, and the correlation between the bubble diameter and the overheated layer thickness was verified.