Heat Fluxes In Excess Research Articles

Simulation of burn control for DEMO using ASTRA coupled with Simulink

DEMO is a proposed demonstration fusion power plant which is currently being designed. Fusion power, Pfus, has to be controlled to stay above a minimum level to produce sufficient net electricity. However, this has an impact on the power through the separatrix, Psep, and thus can produce excessive heat flux to the divertor which can lead to damage. This gives a limit for the maximum allowable Psep.The aim of this work is to develop a framework which will give the possibility to design and to test controllers by simulating DEMO plasma transport. Therefore, we coupled the ASTRA transport modelling code for fusion devices with the Simulink simulation framework. ASTRA is equipped with equilibrium, transport, fuelling, heating and current drive modules. Simulink is a powerful tool to model and to simulate different dynamic systems. It allows fast and simple development of controllers using its built-in blocks. Therefore, coupling of ASTRA with Simulink gives the advantage of fast development of controllers for the power plant modelled with sophisticated physics based on the transport codes.This coupling was used to run the first basic simulations, where we simulated feedback control of fusion power, power through separatrix and pedestal top Greenwald density fraction, nGWped top. nGWped top is controlled via the pellet frequency to keep it below one. Fusion power is controlled with external heating (NBI). To prevent increasing Psep and heat load on the divertor while increasing Pfus, xenon gas can be puffed into the vessel. However, Psep has to be kept above a threshold to stay in H-mode. Therefore, feedback control of the xenon gas puff is modelled. These controllers were used to control fusion power, power through separatrix and pedestal top Greenwald density fraction with two different external disturbances. In the first one, we mimicked changes of the H-factor and in the second one we mimicked the fall of a tungsten flake. Results show that we can control these parameters already with a combination of simple controllers.

How do uncertainties in NCEP R2 and CFSR surface fluxes impact tropical ocean simulations?

NCEP/DOE reanalysis (R2) and Climate Forecast System Reanalysis (CFSR) surface fluxes are widely used by the research community to understand surface flux climate variability, and to drive ocean models as surface forcings. However, large discrepancies exist between these two products, including (1) stronger trade winds in CFSR than in R2 over the tropical Pacific prior 2000; (2) excessive net surface heat fluxes into ocean in CFSR than in R2 with an increase in difference after 2000. The goals of this study are to examine the sensitivity of ocean simulations to discrepancies between CFSR and R2 surface fluxes, and to assess the fidelity of the two products. A set of experiments, where an ocean model was driven by a combination of surface flux components from R2 and CFSR, were carried out. The model simulations were contrasted to identify sensitivity to different component of the surface fluxes in R2 and CFSR. The accuracy of the model simulations was validated against the tropical moorings data, altimetry SSH and SST reanalysis products. Sensitivity of ocean simulations showed that temperature bias difference in the upper 100 m is mostly sensitive to the differences in surface heat fluxes, while depth of 20 °C (D20) bias difference is mainly determined by the discrepancies in momentum fluxes. D20 simulations with CFSR winds agree with observation well in the western equatorial Pacific prior 2000, but have large negative bias similar to those with R2 winds after 2000, partly because easterly winds over the central Pacific were underestimated in both CFSR and R2. On the other hand, the observed temperature variability is well reproduced in the tropical Pacific by simulations with both R2 and CFSR fluxes. Relative to the R2 fluxes, the CFSR fluxes improve simulation of interannual variability in all three tropical oceans to a varying degree. The improvement in the tropical Atlantic is most significant and is largely attributed to differences in surface winds.

Experimental measurements of surface damage and residual stresses in micro-engineered plasma facing materials

The thermomechanical damage and residual stresses in plasma-facing materials operating at high heat flux are experimentally investigated. Materials with micro-surfaces are found to be more resilient, when exposed to cyclic high heat flux generated by an arc-jet plasma. An experimental facility, dedicated to High Energy Flux Testing (HEFTY), is developed for testing cyclic heat flux in excess of 10 MW/m2. We show that plastic deformation and subsequent fracture of the surface can be controlled by sample cooling. We demonstrate that W surfaces with micro-pillar type surface architecture have significantly reduced residual thermal stresses after plasma exposure, as compared to those with flat surfaces. X-ray diffraction (XRD) spectra of the W-(110) peak reveal that broadening of the Full Width at Half Maximum (FWHM) for micro-engineered samples is substantially smaller than corresponding flat surfaces. Spectral shifts of XRD signals indicate that residual stresses due to plasma exposure of micro-engineered surfaces build up in the first few cycles of exposure. Subsequent cyclic plasma heat loading is shown to anneal out most of the built-up residual stresses in micro-engineered surfaces. These findings are consistent with relaxation of residual thermal stresses in surfaces with micro-engineered features. The initial residual stress state of highly polished flat W samples is compressive (≈ -1.3 GPa). After exposure to 50 plasma cycles, the surface stress relaxes to −1.0 GPa. Micro-engineered samples exposed to the same thermal cycling show that the initial residual stress state is compressive at (- 250 MPa), and remains largely unchanged after plasma exposure.

Mechanism of seasonal Arctic sea ice evolution and Arctic amplification

Abstract. Sea ice loss is proposed as a primary reason for the Arctic amplification, although the physical mechanism of the Arctic amplification and its connection with sea ice melting is still in debate. In the present study, monthly ERA-Interim reanalysis data are analyzed via cyclostationary empirical orthogonal function analysis to understand the seasonal mechanism of sea ice loss in the Arctic Ocean and the Arctic amplification. While sea ice loss is widespread over much of the perimeter of the Arctic Ocean in summer, sea ice remains thin in winter only in the Barents–Kara seas. Excessive turbulent heat flux through the sea surface exposed to air due to sea ice reduction warms the atmospheric column. Warmer air increases the downward longwave radiation and subsequently surface air temperature, which facilitates sea surface remains to be free of ice. This positive feedback mechanism is not clearly observed in the Laptev, East Siberian, Chukchi, and Beaufort seas, since sea ice refreezes in late fall (November) before excessive turbulent heat flux is available for warming the atmospheric column in winter. A detailed seasonal heat budget is presented in order to understand specific differences between the Barents–Kara seas and Laptev, East Siberian, Chukchi, and Beaufort seas.

Thermal design of helium cooled divertor for reliable operation

Nuclear fusion is the promising energy sources because the fuels for power generation are abundant and by-products are eco-friendly rather than other power generations. However, there are several conundrums that must be solved for developing the nuclear fusion plants, including DEMO (DEMOnstration nuclear fusion reactors), such as superconducting system, blanket, cryostats and others. In particular, a divertor is one of essential components because of withstanding the excessive heat flux (∼10MW/m2) from the high-temperature plasma. Therefore, it is necessary for thermal design of divertor module under tremendous heat flux to develop the nuclear fusion plants. We investigate thermophysical behavior by convective heat transfer and suggest principle operating variables to decide for appropriate thermal design for divertor module. Based on the simplified thermal circuit, we demonstrate that the observed correlation can predict thermophysical characteristics of the divertor module and present the prerequisites for reliable thermal design based on the thermal design maps in terms of principle operating variables of divertor module. Finally, we reveal that the safety factor of thimble is primary concern to establish thermal design of divertor module. The present study will contribute to the development of divertor in nuclear fusion plants and the applications for high heat flux and temperature devices.

Two-dimensional hexagonal boron nitride as lateral heat spreader in electrically insulating packaging

The need for electrically insulating materials with a high in-plane thermal conductivity for lateral heat spreading applications in electronic devices has intensified studies of layered hexagonal boron nitride (h-BN) films. Due to its physicochemical properties, h-BN can be utilised in power dissipating devices such as an electrically insulating heat spreader material for laterally redistributing the heat from hotspots caused by locally excessive heat flux densities. In this study, two types of boron nitride based heat spreader test structures have been assembled and evaluated for heat dissipation. The test structures separately utilised a few-layer h-BN film with and without graphene enhancement drop coated onto the hotspot test structure. The influence of the h-BN heat spreader films on the temperature distribution across the surface of the hotspot test structure was studied at a range of heat flux densities through the hotspot. It was found that the graphene-enhanced h-BN film reduced the hotspot temperature by about 8–10 °C at a 1000 W cm−2 heat flux density, a temperature decrease significantly larger than for h-BN film without graphene enhancement. Finite element simulations of the h-BN film predict that further improvements in heat spreading ability are possible if the thermal contact resistance between the film and test chip are minimised.

Direct numerical simulation of gas transfer across the air–water interface driven by buoyant convection

A series of direct numerical simulations of mass transfer across the air–water interface driven by buoyancy-induced convection have been carried out to elucidate the physical mechanisms that play a role in the transfer of heat and atmospheric gases. The buoyant instability is caused by the presence of a thin layer of cold water situated on top of a body of warm water. In time, heat and atmospheric gases diffuse into the uppermost part of the thermal boundary layer and are subsequently transported down into the bulk by falling sheets and plumes of cold water. Using a specifically designed numerical code for the discretization of scalar convection and diffusion, it was possible to accurately resolve this buoyant-instability-induced transport of atmospheric gases into the bulk at a realistic Prandtl number ($\mathit{Pr}=6$) and Schmidt numbers ranging from$\mathit{Sc}=20$to$\mathit{Sc}=500$. The simulations presented here provided a detailed insight into instantaneous gas transfer processes. The falling plumes with highly gas-saturated fluid in their core were found to penetrate deep inside the bulk. With an initial temperature difference between the water surface and the bulk of slightly above$2$ K, peaks in the instantaneous heat flux in excess of$1600~\text{W}~\text{m}^{-2}$were observed, proving the potential effectiveness of buoyant-convective heat and gas transfer. Furthermore, the validity of the scaling law for the ratio of gas and heat transfer velocities$K_{L}/H_{L}\propto (\mathit{Pr}/\mathit{Sc})^{0.5}$for the entire range of Schmidt numbers considered was confirmed. A good time-accurate approximation of$K_{L}$was found using surface information such as velocity fluctuations and convection cell size or surface divergence. A reasonable time accuracy for the$K_{L}$estimation was obtained using the horizontal integral length scale and the root mean square of the horizontal velocity fluctuations in the upper part of the bulk.

The Rainfall Annual Cycle Bias over East Africa in CMIP5 Coupled Climate Models

Abstract East Africa has two rainy seasons: the long rains [March–May (MAM)] and the short rains [October–December (OND)]. Most CMIP3/5 coupled models overestimate the short rains while underestimating the long rains. In this study, the East African rainfall bias is investigated by comparing the coupled historical simulations from CMIP5 to the corresponding SST-forced AMIP simulations. Much of the investigation is focused on the MRI-CGCM3 model, which successfully reproduces the observed rainfall annual cycle in East Africa in the AMIP experiment but its coupled historical simulation has a similar but stronger bias as the coupled multimodel mean. The historical–AMIP monthly climatology rainfall bias in East Africa can be explained by the bias in the convective instability (CI), which is dominated by the near-surface moisture static energy (MSE) and ultimately by the MSE’s moisture component. The near-surface MSE bias is modulated by the sea surface temperature (SST) over the western Indian Ocean. The warm SST bias in OND can be explained by both insufficient ocean dynamical cooling and latent flux, while the insufficient shortwave radiation and excess latent heat flux mainly contribute to the cool SST bias in MAM.

Microcontact-Enhanced Thermoelectric Cooling of Ultrahigh Heat Flux Hotspots

The dissipated power of insulated gate bipolar transistor and high electron mobility transistor amplifiers is typically nonuniform, resulting in areas of elevated temperature, or hotspots, which can have very large heat fluxes, on the order of 1000 W/cm2. While various bulk cooling systems are being researched to remove large amounts of heat, they uniformly reduce the chip temperature, leaving the temperature nonuniformity. Therefore, advanced hotspot cooling techniques, which provide localized cooling, are also required to unlock the full potential of cutting edge power devices. Thermoelectric coolers have previously been demonstrated as an effective method of producing on-demand cooling for the removal of localized hotspots. However, the heat flux of the hotspots that can be cooled is limited by the maximum cooling flux of thermoelectric devices. This paper demonstrates the thermal and reliability performance of a microcontact-enhanced thermoelectric cooling configuration, which uses a contact structure etched directly out of the electronic substrate to concentrate the cooling produced by a commercially available thermoelectric module. The 22 K of cooling, resulting in a hotspot temperature rise of <6 K for a heat flux of 2.5 kW/cm2, was experimentally demonstrated using a Laird HV37 thin-film thermoelectric module with a maximum device level cooling flux of 66 W/cm2. A numerical model was created, and it is predicted that when the chip and microcontact geometry is optimized, hotspots with heat fluxes in excess of 3 kW/cm2 can be cooled by nearly 40 K, reducing the hotspot temperature rise to 0 K.

A quantitative model for heat pulse propagation across large helical device plasmas

It is known that rapid edge cooling of magnetically confined plasmas can trigger heat pulses that propagate rapidly inward. These can result in large excursion, either positive or negative, in the electron temperature at the core. A set of particularly detailed measurements was obtained in Large Helical Device (LHD) plasmas [S. Inagaki et al., Plasma Phys. Controlled Fusion 52, 075002 (2010)], which are considered here. By applying a travelling wave transformation, we extend the model of Dendy et al., Plasma Phys. Controlled Fusion 55, 115009 (2013), which successfully describes the local time-evolution of heat pulses in these plasmas, to include also spatial dependence. The new extended model comprises two coupled nonlinear first order differential equations for the (x, t) evolution of the deviation from steady state of two independent variables: the excess electron temperature gradient and the excess heat flux, both of which are measured in the LHD experiments. The mathematical structure of the model equations implies a formula for the pulse velocity, defined in terms of plasma quantities, which aligns with empirical expectations and is within a factor of two of the measured values. We thus model spatio-temporal pulse evolution, from first principles, in a way which yields as output the spatiotemporal evolution of the electron temperature, which is also measured in detail in the experiments. We compare the model results against LHD datasets using appropriate initial and boundary conditions. Sensitivity of this nonlinear model with respect to plasma parameters, initial conditions, and boundary conditions is also investigated. We conclude that this model is able to match experimental data for the spatio-temporal evolution of the temperature profiles of these pulses, and their propagation velocities, across a broad radial range from r/a≃0.5 to the plasma core. The model further implies that the heat pulse may be related mathematically to soliton solutions of the Korteweg-de Vries-Burgers equation.

Development of multi-layered thermal protection system (TPS) for aerospace applications

Multi-layered UHTC composites for use in an extreme oxidation environment were fabricated by co-sintering. The multi-layered composite system consists of three layers and was designed with (i) an outer surface with the capacity to withstand heat fluxes in excess of 25 MW m−2, (ii) an intermediate layer with the ability to curtail diffusion of O2 and (iii) a bottom layer with better performance at high temperatures. One design has MeB2 (where Me = Zr or Hf) as the outer or top layer, MeCxOy as the intermediate layer and MeB2/SiC as the bottom layer. Since little is known about MeCxOy ceramics, a detailed study has been carried out to synthesise and characterise them focussed on controlling MeCxOy stoichiometry. During co-sintering, the outer MeB2 layer thickness played a crucial role in providing a crack-free component. Indicative calculations of the residual stresses confirm that a compressive stress due to bending of the disks can stop crack growth through the MeB2 layer when its thickness is increased to 3 mm, and that the tensile stress in the bottom layer is also reduced. The cross-section microstructure of the optimised multi-layered UHTC composite shows a crack-free seamless interface.

Comparison of measurements and computations of isothermal flow velocity inside HyperVapotrons

HyperVapotrons are two-phase water-cooled heat exchangers able to receive high heat fluxes (HHF) by employing a cyclic phenomenon called the “Vapotron Effect”. HyperVapotrons have been used routinely in HHF nuclear fusion applications. A detailed experimental investigation on the effect giving rise to the ability to sustain steady state heat fluxes in excess of 10MW/m2 has not yet been possible and hence the phenomenon is not yet well understood. The coolant flow structures that promote the effect have been a major point of interest, and many investigations based on computational fluid dynamic (CFD) simulations have been performed in the past. The understanding of the physics of the coolant flow inside the device may hold the key to further optimisation of engineering designs. However, past computational investigations have not been experimentally evaluated. Isothermal flow velocity distribution measurements of the fluid flow in HyperVapotron optical models with high spatial resolution are performed in this paper. The same measurements are subsequently calculated via commercial CFD software. The isothermal CFD calculation is compared to the experimental velocity measurements to deduce the accuracy of the CFD investigations carried out. This unique comparison between computational and experimental results in HyperVapotrons will direct future efforts in analysing similar devices.

Impact of upper ocean processes and air-sea fluxes on seasonal SST biases over the tropical Indian Ocean in the NCEP Climate Forecasting System

The present study aims to examine the role of air–sea interactions and upper ocean processes in determining the tropical Indian Ocean (TIO) seasonal sea surface temperature (SST) bias in Climate Forecast System version 1 (CFSv1) and version 2 (CFSv2) free runs. CFSv1 displayed dipole like east–west SST bias over the equatorial Indian Ocean from boreal summer to winter and is consistent with errors (bias) in surface winds and upper ocean advection. Large zonal gradients in sea level pressure (SLP) bias and the associated surface wind biases are primarily responsible for the upper ocean current bias. However, over the southern Indian Ocean and parts of Arabian Sea, strong bias in heat flux and mixed layer depth (MLD) have mainly contributed for the SST biases in CFSv1. Equatorial current system is better represented in CFSv2 compared to CFSv1. Improvement in the representation of land-surface processes appears to be contributing towards improving atmospheric circulation and SLP gradients in CFSv2, which may be responsible for the improved ocean circulation. Importantly, east–west dipole like SST bias prevalent in CFSv1 is absent in CFSv2. However, there is a prominent systematic basin-wide TIO cold SST bias in CFSv2. Large biases in surface heat flux (net negative bias) and MLD (deeper) are mainly responsible for SST biases in CFSv2. Negative net heat flux bias in CFSv2 is primarily due to specific humidity bias-induced excess latent heat flux (LHF). Deepening of MLD is mainly due to strong convective mixing, a resultant of anomalous LHF release, which in turn leads to negative SST bias. Models comparison reveals that although representation of SST in CFSv2 is better than in CFSv1, it is essential to improve further the equatorial ocean dynamics and off-equatorial thermodynamics in the form of moist processes and radiative parameterization in order to reduce SST bias in CFSv2.

軒下部に衝突する火炎の熱伝達特性

A series of model experiment was conducted for evaluating heat transfer to a non-flammable eave during flame impingement. Excess temperature ΔT and incident heat flux to the eave q″ were measured under sixteen experimental runs with varying parameters of eave section geometry, burner geometry, installation position of the burner, and fuel supply rate. In order to correlate the maximum excess temperature ΔTmax , a reference length scale L was deduced from the governing dimensionless parameter Q*. Dimensional analysis of thermal structure of the flow showed that the maximum excess temperature ΔTmax is expected to be proportional to ((x +H)/L)0 in the flaming regime, and ((x +H)/L)-1 in the non-flaming regime. The agreement with the experimental data was reasonable for all the tested eave sections and burner geometries.

Effects of fire-fighting on a fully developed compartment fire: Temperatures and emissions

This study evaluates the effects and consequences of fire-fighting operations on the main characteristics of a fully-developed compartment fire. It also presents data and evaluation of the conditions to which fire-fighters are exposed. A typical room enclosure was used with ventilation through a corridor to the front access door. The fire load was wooden pallets. Flashover was reached and the fire became fully developed before the involvement of the fire-fighting team. The progression of the fire-fighters through the corridor and the main-room suppression attack – in particular the effect of short, medium and long water pulses on either the hot gas layer or the fire seat – was charted against the compartment temperatures, heat release rates, oxygen levels and toxic species concentrations. The fire fighting team was exposed to extreme conditions, heat fluxes in excess of 35kW/m2 and temperatures of the order of 250°C even at crouching level. The fire equivalence ratio showed rich burning with high toxic emissions in particular of CO and unburnt hydrocarbons very early in the fire history and a stabilisation of the equivalence ratio at about 1.8. The fire fighting operations made the combustion temporarily richer and the emissions even higher.

Modeling and Analysis of the W7-X High Heat-Flux Divertor Scraper Element

The Wendelstein 7-X stellarator experiment is scheduled for the completion of device commissioning and the start of first plasma in 2015. At the completion of the first two operational phases, the inertially cooled test divertor unit will be replaced with an actively cooled high heat-flux divertor, which will enable the device to increase its pulse length to steady-state plasma performance. Plasma simulations show that the evolution of bootstrap current in certain plasma scenarios produce excessive heat fluxes on the edge of the divertor targets. It is proposed to place an additional scraper element in the 10 divertor locations to intercept some of the plasma flux and reduce the heat load on these divertor edge elements. Each scraper element may experience a 500-kW steady-state power load, with localized heat fluxes as high as 20 MW/m2. Computational analysis has been performed to examine the thermal integrity of the scraper element. The peak temperature in the carbon-carbon fiber composite, the total pressure drop in the cooling water, and the increase in water temperature must all be examined to stay within specific design limits. Computational fluid dynamics modeling is performed to examine the flow paths through the multiple monoblock fingers as well as the thermal transfer through the monoblock swirl tube channels.

A theoretical and numerical investigation of travelling wave induction microfluidic pumping in a temperature gradient

The phenomenon of induction electrohydrodynamics (EHD) has recently received great attention as a promising driving mechanism for microfluidic pumping due to its miniaturization capability. To obtain a high working efficiency of induction micropumps, a vertical temperature gradient can be imposed along the depth of a pump channel. A travelling wave (TW) potential signal propagating along an electrode array at the channel substrate interacts with this conductive heat flux, resulting in a local free charge distribution inside the bulk fluid. The induced charge wave lags behind the voltage wave in the spatial phase, and this out-of-phase polarization based pumping effect exhibits a single structural dispersion at charge relaxation frequency of the dielectric system. The classical model of electrothermal flow has always been used to numerically obtain the flow field of TW pumps, but the effect of its small temperature gradient approximation has rarely been investigated. In this study, an enhanced treatment for induction EHD modelling is developed, in which the deflection of potential contour lines caused by large temperature gradients is successfully characterized by an advection–diffusion equation, and a more accurate expression of electrothermal body force is derived and introduced to fluid dynamics as a source term of electrical origin. For the calculation of a repulsion-type induction micropump, although both models present similar results in a small thermal gradient, the enhanced one can provide more exact frequency-dependence of the pump performance and spatial distribution of electrostatic force as well as the resulting velocity profile in an excessive heat flux. Furthermore, a model extension for Joule heating induced TW pumping is also presented, and surprisingly matches the unexpected nonlinear fluid flow behaviour at higher conductivities as reported in a pioneering literature. These results can provide valuable insights into induction pumping of lab-on-chip microfluidic samples.

How much net surface heat flux should go into the Western Pacific Warm Pool?

mean of 18 Wm � 2 for the five-member low net heat flux group (ERA-Interim, CORE.2, NCEP 1 and 2, and ERA-40) and of 49 Wm � 2 for the four-member high net heat flux group (CFSR, OAFluxþISCCP, NOCSv2.0, and MERRA). This study used a pool-area based heat budget analysis together with in situ air-sea and subsurface measurements to examine the physical consistency of the nine flux climatologies and to ascribe the statistical uncertainty of each product. The heat budget analysis indicates that the annual mean net surface heat flux should be 28 6 10 Wm � 2 . The observed eddy coefficient along the 28 � C isotherm is 1.5 cm 2 s � 1 based on the TAO/TRION buoys and the historical records. The ocean cannot dissipate the excessive high heat fluxes, while the low fluxes cannot balance the estimated diffusive heat flux across the isotherm. Both the one-point direct comparison and pool integrated eddy diffusive heat flux analysis demonstrate that, the high net heat flux climatologies have high bias; on the other hand, the low fluxes have low bias. These biases and uncertainties are given and documented in this paper.

Radial Microchannel Reactors (RMRs) for Efficient and Compact Steam Reforming of Methane: Experimental Demonstration and Design Simulations

This paper provides a first report of a novel radial microchannel reactor (RMR) architecture for catalytic steam reforming of methane to hydrogen and/or synthesis gas. A bench-scale RMR prototype with a channel width of 0.700 mm and wash-coated with 0.030 mm of a Ni-based catalyst is operated at 750 °C and 11 bar with feed composition of 25 mol % CH4, balance H2O. Results indicate that volumetric heat fluxes in excess of 150 W cm–3 are achievable at residence times of 6.8 ms, while volumetric heat fluxes in excess of 100 W cm–3 are observed at overall thermodynamic efficiencies >70%. Computational fluid dynamic (CFD) simulations of the RMR design are demonstrated for accurately predicting methane steam reformer performance and, subsequently, for rapidly investigating the influence of microchannel dimensions upon RMR performance. Design simulations predict that 20–100% improvements in volumetric heat-transfer rates may be achieved by reducing channel widths from 0.700 to 0.300 mm.

Comparison of Deionized Water and FC-72 in Pool and Jet Impingement Boiling Thermal Management

High heat fluxes and stringent constraints on surface temperature and its uniformity during thermal management of electrical and electronic components often necessitate use of boiling heat transfer. This paper compares the pool and jet impingement boiling heat transfer characteristics of deionized water and FC-72 at an equivalent fluid saturation temperature of 57°C and for identical experimental conditions. To lower the saturation temperature of water down to 57°C, experiments with water are performed at a reduced absolute system pressure of 0.176 bar. Despite the reduction in pressure, pool boiling critical heat flux with deionized water is found to be 3.6 times larger than with FC-72. Furthermore, jet impingement is seen to enhance boiling heat transport more significantly for water than for FC-72. Consequently, heat transfer coefficients during jet impingement boiling are as much as 3.9 times larger for water compared to FC-72 at identical Reynolds numbers and surface temperatures. The heat transfer advantage of using water is mainly associated with the superior thermophysical properties of this fluid. However, in addition to the large fluid saturation temperature at atmospheric conditions, direct cooling of electronics is frequently not possible using water due to the incompatibility of the fluid with electrical components. To assess the practical utility of subatmospheric deionized water through indirect cooling of electronics, a 1-D heat sink analysis is performed on a multichip module geometry. The overall thermal resistance of the heat sink using water is determined to be around two times lower than that of direct cooling of FC-72 on a silicon substrate. Under saturation condition of the working fluids, dissipation of heat fluxes in excess of ~45 W/cm2 with the multichip module using water is constrained by the chip surface temperature limit.