Heat Flux Levels Research Articles

Flat and U-Shaped Heat Spreaders for High-Power Electronics

To achieve a high heat-flux level and reduce manufacturing costs associated with conventional heat pipes, the concept of network heat spreaders employing a boiling heat-transfer mechanism in a narrow space had been proposed, and several flat-plate wickless heat spreaders had been designed and fabricated. The heat spreaders had been tested under different working conditions and orientations relative to gravity with very good results. The previously tested network heat spreaders, however, were based on plates with a relatively large size for general heat spreading purposes. In the present study, network heat spreaders with overall dimensions of 78 2 62 2 3.2 mm are designed and fabricated. Spreaders of this size are intended for use as heat sinks of high-power electronic components. External cooling fins are attached to enhance air-cooling heat transfer rate. The network heat spreaders are tested under various working conditions with water as the working fluid. The maximum heat input rate achieved is about 150 W with a corresponding heat flux of 60 W/cm 2 . Compared to the performance of a solid copper plate having the same overall size as the spreader, the maximum temperature difference over the surface is reduced from about 32°C to 3.3°C. The heat transfer performance of the spreader is also largely dependent on the filling ratio of the working fluid and the boiling heat transfer in the narrow space. For these reasons, boiling heat transfer mechanisms in a narrow space are analyzed, and a spreader design that would improve the performance in a horizontal position is described.

Validation of Heat-Flux Predictions on the Outer Air Seal of a Transonic Turbine Blade

It is desirable to accurately predict the heat load on turbine hot section components within the design cycle of the engine. Thus, a set of predictions of the heat flux on the blade outer air seal of a transonic turbine is here validated with time-resolved measurements obtained in a single-stage high-pressure turbine rig. Surface pressure measurements were also obtained along the blade outer air seal, and these are also compared to three-dimensional, Reynolds-averaged Navier-Stokes predictions. A region of very high heat flux was predicted as the pressure side of the blade passed a fixed location on the blade outer air seal, but this was not measured in the experiment. The region of high heat flux was associated both with very high harmonics of the blade-passing event and a discrepancy between predicted and measured time-mean heat-flux levels. Further analysis of the predicted heat flux in light of the experimental technique employed in the test revealed that the elevated heat flux associated with passage of the pressure side might be physical. Improvements in the experimental technique are suggested for future efforts.

Improved Method For Analyzing Ignition Data From The Cone Calorimeter In The Vertical Orientation

Extensive research has been conducted on the use of the Cone Calorimeter to obtain flammability properties of materials for modeling. Ignition properties such as surface temperature at ignition (Tig) and thermal inertia (kρc) are typically determined on the basis of piloted ignition times measured at different heat flux levels. A specific functional relationship between piloted ignition time and heat flux is used to correlate the data and estimate the material properties. The functional relationship is based on the solution of a simplified form of the heat conduction problem in the Cone Calorimeter. Previous ignition studies have always assumed that the surface heat losses involve Newtonian cooling, which is characterized by a constant convection coefficient. However, earlier measurements obtained by the author show that the convection coefficient in the Cone Calorimeter for specimens in the vertical orientation could be more appropriately expressed as a quadratic function of the heat flux. This paper presents the experimental data and analysis that resulted in this proposed quadratic relationship. The solution of the modified conduction model is derived, and its use for the analysis of ignition data is illustrated for six solid wood species.

The Role of Mould Fluxes in Continuous Casting-So Simple Yet So Complex

The performance of mould fluxes in providing the optimum levels of lubrication and horizontal heat flux is relatively simple since they rely on only three properties, the viscosity and break temperature of the flux and the crystalline fraction developed in the slag film. These parameters can be readily calculated according to the mould dimensions, casting conditions and steel grade. It is only when the flux is used to combat other problems, such as those arising from turbulent metal flow (e.g. slag entrapment) that flux behaviour becomes complex and more difficult to understand. Some recent developments may help to combat problems associated with turbulent metal flow and free the flux to carry out its principal duties of providing optimum lubrication and horizontal heat flux.

樹木の発炎燃焼性に関連する未燃ガスの発生について

It was confirmed by the existing studies that, when it was exposed by only radiant heat, leaves of many species of trees ignited without flame but Juniperus chinensis cv. Kaizuka flamed lower than the tolerance limit of radiant heat. (It is said the lowest heat flux level of glowing as to coniferous trees was 13.95kW/m2.) The research on such danger is not progressing but which should be verified, for utilizing the fire prevention function of trees and for planning landscape in narrow residence, such as city. So, in this experiments, the concentration and transition of unburned combustible gas and the temperature under the leaves were measured about 23 species of trees, in this time the hydrocarbon was mainly measured as the unburned gas. The following results were obtained: (1) The states of hydrocarbon generating and temperature transition were obviously different depending on the species. (2) Three species of the family Cupressaceae discharged much hydrocarbon from the first stage of heating as well as J. chinensis cv. Kaizuka. (3) Aucuba japonica, Prunus mume, Phyllostachys bambusoides, and Pleioblastus chino generated hydrocarbon few from beginning to end. (4) Viburnum awabuki, Ginkgo biloba, and Hydrangea macrophylla generated few in first stage compared with the total quantity.

Enhanced boiling heat transfer in restricted spaces of a compact tube bundle with enhanced tubes

In flooded-type tube bundle evaporators with smooth tubes and general tube gaps, both wall superheat and heat flux are generally quite low and boiling cannot occur on the heated tubes. But when the tube gap is quite small or the enhanced heat transfer tubes are employed, the incipient boiling can occur at low heat flux levels and results in a significant heat transfer enhancement effect. This study investigates experimentally enhancement effects by the restricted space comprising the compact tube bundle and the enhanced tubes for boiling heat transfer of pure water and salt-water mixtures under atmospheric pressure. The experimental results show that the small tube gaps can greatly enhance boiling heat transfer for the compact enhanced tube bundle.

Mechanisms of steady-state nucleate pool boiling in microgravity.

Research on nucleate pool boiling in microgravity using R-113 as a working fluid was conducted using a five-second drop tower and five space flights at a/g approximately 10(-4). A 19 x 38-mm flat gold film heater was used that allowed cine camera viewing both from the side and the bottom of the heater. It was concluded that for both subcooled and saturated liquids long-term steady-state pool boiling can take place in reduced gravity, but the effectiveness of the boiling heat transfer appears to depend on the heater geometry and on the size and the properties of fluids. Heat transfer is enhanced at lower heat flux levels and the CHF increases as the subcooling increases. It was found that several mechanisms are responsible for the steady-state nucleate pool boiling in the absence of buoyancy. The mechanisms considered here are defined and summarized as bubble removal, bubble coalescence, thermocapillary flow, bubble migration, and latent heat transport.

Analysis of External Cooling of the Reactor Vessel During Severe Accidents

An analysis is presented of the integral behavior of the external cooling of a reactor vessel by natural circulation during a severe accident to investigate the feasibility of the in-vessel retention strategy for a high-power reactor by using the RELAP5/MOD3 computer code. It is shown that two-phase flow instability phenomena, including natural-circulation oscillation and density wave oscillations, affect the local thermal margin at the reactor vessel wall. The heat load on the reactor vessel is simplified as a uniform heat flux load of 600 kW/m2 in the base case. A sensitivity study for the effect of the inlet K factor, nonuniform heat flux distribution, inlet flow area, and subcooling of the pool water is performed to evaluate the local thermal margin. The results of the analysis show that natural-circulation cooling is marginal at this level of heat flux. It also clearly indicates that a system level of analysis for two-phase natural circulation, including the sensitivity study on the design parameters, is necessary to ensure successful implementation of the external cooling.

Large eddy simulation of the turbulent flow past a backward-facing step with heat transfer and property variations

The heat transfer and fluid mechanics of a turbulent separating and reattaching flow past a backward-facing step are studied using large eddy simulation. A fully coupled, collocated-grid, low-Mach number preconditioned, compressible formulation employing central differences was developed to conduct the simulations. A sixth order compact filter was used to prevent pressure–velocity decoupling. A dynamic sub-grid scale procedure employing the compressible version of the Smagorinsky model has been used to capture the effects of the small scales. The isothermal turbulent flow past the step, at a Reynolds number of 5540 (based on the step height and upstream centerline velocity) and a Mach number of 0.006, was simulated to validate the formulation. Subsequently, the bottom wall downstream of the step was supplied with a uniform wall heat flux; three simulations with increasing heat flux levels were conducted. The viscous sub-layer played a critical role in controlling the heat transfer rate. Streamwise and wall-normal turbulent heat fluxes were of the same order of magnitude. The Reynolds analogy did not hold in the recirculation region. However, the Stanton number profiles showed a striking similarity with the fluctuating skin-friction profiles.

Prediction of heat release in the single burning item test

AbstractThis paper presents a modification of an existing mathematical model that uses cone calorimeter test results to predict heat release rate in the single burning item (SBI) test. A method for classification of cases based on multivariate statistical analysis is incorporated in the modified model. This makes it possible to determine the development of the heat release curve in the SBI test with better precision than the original version was able to. The model has been applied to 100 cases of cone calorimeter test results from 33 different products. For most of the products the predicted shape and level of the heat release curves are close to measurements in the SBI test. Using the predicted heat release results as input to calculation of FIGRA0.2MJ and THR600s within the new European system for reaction‐to‐fire classification, we were able to predict membership of the correct class in 90% of the analysed cases. Copyright © 2002 John Wiley & Sons, Ltd.

Experimental and numerical study of pressure drop and heat transfer in a single-phase micro-channel heat sink

The pressure drop and heat transfer characteristics of a single-phase micro-channel heat sink were investigated both experimentally and numerically. The heat sink was fabricated from oxygen-free copper and fitted with a polycarbonate plastic cover plate. The heat sink consisted of an array of rectangular micro-channels 231 μm wide and 713 μm deep. Deionized water was employed as the cooling liquid and two heat flux levels, q″ eff =100 W/cm 2 and q″ eff =200 W/cm 2 , defined relative to the planform area of the heat sink, were tested. The Reynolds number ranged from 139 to 1672 for q″ eff =100 W /cm 2 , and 385 to 1289 for q″ eff =200 W /cm 2 . The three-dimensional heat transfer characteristics of the heat sink were analyzed numerically by solving the conjugate heat transfer problem involving simultaneous determination of the temperature field in both the solid and liquid regions. Also presented and discussed is a detailed description of the local and average heat transfer characteristics of the heat sink. The measured pressure drop and temperature distributions show good agreement with the corresponding numerical predictions. These findings demonstrate that the conventional Navier–Stokes and energy equations can adequately predict the fluid flow and heat transfer characteristics of micro-channel heat sinks.

Falling film evaporation heat transfer of water/salt mixtures from roll-worked enhanced tubes and tube bundle

An experimental study is carried out for enhancement of falling film evaporation heat transfer of pure water and water/salt mixtures on horizontal smooth tube and two kinds of structured tube bundles under atmospheric pressure. The experimental results show that the low-cost roll-worked tube can greatly enhance the evaporation heat transfer performance of the falling film, and make it comparable to that of expensive commercial enhanced tubes such as GEWA-T tubes, TE tubes and HF tubes, even at low and moderate heat flux levels. The average evaporation heat transfer coefficients for the roll-worked tube bundle are basically independent from the parameters tested such as flow and heating conditions, salt-concentrations, as well as geometries of the tube bundles. The present experimental data result in a constant heat transfer coefficient; α≈20 kW/m 2 K, in the convective heat transfer range of the heat fluxes <10 5 W/m 2.

Effects of diesel/water emulsion on heat flow and thermal loading in a precombustion chamber diesel engine

An experimental investigation has been carried out to study the effects of using water/diesel emulsion fuel in an indirect injection diesel engine on the heat flux crossing liner and cylinder head, thermal loading and metal temperature distribution. A single cylinder precombustion chamber diesel engine has been used in the present work. The engine was instrumented for performance, metal temperature and heat flux measurements. The pure gas oil fuel and different ratios of water/diesel emulsion were used and their effects on the heat flux level and the injector tip temperature are studied. Two correlation were found for the heat flux crossing the liner and the cylinder head at various water/diesel emulsion ratios, fuelling rate and thermocouple probe locations. It was found that the addition of water to diesel fuel, to control the nitrogen oxides emissions, has great influence on reducing the heat flux, the metal temperatures and thermal loading of combustion chamber components.

Developing 3-D turbulent mixed convection in a circular duct inserted with longitudinal strips

A numerical study was undertaken for developing turbulent mixed convection in a horizontal circular tube with strip-type inserts (longitudinal strip LS and crossed-strip CS inserts) for 6500⩽Re⩽19 500 and 0⩽ Gr⩽10 8. The turbulence model used is a standard version of k– ε model with wall functions. Three-dimensional Navier–Stokes equations were solved using some modifications to the SIMPLE solution procedure. Temperature data and heat transfer coefficients at different heat flux levels were calculated and correlated in terms of relevant parameters. The buoyancy effect was examined in detail for 0⩽ Gr/ Re 2⩽2.37. It is found that the buoyancy effect has less influence for tubes with inserts compared to the bare tubes. Moreover, the heat transfer enhancement was found to be 2–3 times that of bare tubes. The results were also compared with those from the corresponding previous experimental data. A good agreement was found.

An Interferometric Study of Free Convection at a Window Glazing with a Heated Venetian Blind

The free convective heat transfer rate from a vertical isothermal plate adjacent to a heated aluminum venetian blind has been measured using a Mach-Zehnder interferometer. The blind was heated electrically to simulate solar heating, and the plate was heated above the room temperature to simulate an indoor window glazing. Local and average heat transfer data were obtained for three blind heat flux levels, two blind-to-plate spacings, and four slat angles in the laminar flow regime. For most cases, data were obtained up to a plate Rayleigh number of Ra L = 2.8 × 106 based on the distance from the leading edge. Infinite fringe interferograms were taken for temperature field visualization. The results show that blind heat flux has a strong influence on local and average heat transfer coefficients on the plate. It was also found that the spacing between the inner tips of the slats and the plate had a substantial impact on the convection. For close spacings, the heated slats were found to produce a strong periodic variation in the local heat transfer distribution on the plate. For all cases studied, as the blind heat flux increased, the convective heat transfer rate from the plate decreased dramatically. In addition, as the blind heat flux increased, the average convection coefficient became more dependent on both the blind-to-plate spacing and slat angle.

Heat Transfer of Confined Circular Jet Impingement

ABSTRACTExperiments for heat transfer characteristics of confined circular single jet impingement were conducted. The effect of jet Reynolds number, jet hole-to-plate spacing and heat flux levels on heat transfer characteristics of the heated target surface was examined and presented. The local heat transfer coefficient along the surface is measured and correlations of the stagnation point, local and average Nusselt number are developed and discussed. Finally, comparisons of the present data with existing results were also made.

Modeling the Nukiyama Curve for Water-Cooled Fusion Divertor Channels

A conclusive safety assessment of a fusion reactor requires that the thermal response of the divertor assembly is known with a high degree of accuracy. Such accuracy is mandated because the divertor assembly is subjected to the highest levels of incident heat flux within the reactor. In order to accurately predict the thermal response of the divertor's cooling channels, it is necessary to have a complete model of the Nukiyama boiling curve for the water conditions of interest. Currently published models of the Nukiyama curve for fusion divertor channels have only included the regimes of forced convection, partially and fully developed nucleate boiling, and the local CHF. This paper presents a model that includes these pre-CHF regimes and the post-CHF regime of transition boiling. The model is unique because (1) it tightly integrates the respective heat transfer correlations and makes heat transfer predictions for the water conditions and incident heat fluxes that are fusion-specific, (2) predicts post-CHF heat transfer properties for a swirl tape divertor channel, and (3) validates its predictions via comparison with experimental data. Based on these three points, this model is considered as one of the best available methods for predicting the Nukiyama curve for a water-cooled fusion device.

Measurements of Three-Dimensional, Local Subcooled Flow Boiling Heat Flux and Related Critical Heat Flux for PFCs

The optimized design of one-side-heated plasma-facing components (PFC) is dependent on knowing the local distribution of inside wall heat flux in the flow channels. The local inside wall heat flux can be obtained from selectively chosen local PFC wall temperatures close to the inside boundary of the flow channel. To this end, three-dimensional thermal measurements for a one-side-heated monoblock were made and show: (1) the three-dimensional variation of the wall temperature close to both the heated and fluid-solid surface boundaries, (2) the resultant effects of local subcooled flow boiling on the 3-D wall temperature/outside heat flux relationship – one of which is the 3-D wall temperature profile is almost unchanged in the vicinity for incident heat flux levels between the onset to fully developed boiling and CHF, and (3) the occurrence of local CHF and local post-CHF. The monoblock has a 180.0 mm heated length, has a 10.0 mm inside diameter, and has a circular-like cross-section with a 30.0 mm nominal outside diameter.

BUBBLE BEHAVIOR AND HEAT TRANSFER MECHANISM IN FC-72 POOL BOILING

A transparent heater made of a thin synthetic diamond substrate along with a high-speed camera was used to investigate bubble behavior during pool boiling. The heater design, combined with the selected FC-72 liquid, overcame the difficulty of previous thin-film heater experiments where transparency and adequate heat flux could not be simultaneously achieved. It also resulted in an essentially uniform temperature field over the heater surface. The growth and merging of bubbles were visualized and quantitatively documented. The relative contribution from phase change to the overall heat flux was determined at several heat flux levels. At a heat flux level half of the critical heat flux (CHF), surface bubble nucleation was found to contribute to more than 70% of the heat transfer from the heater surface. At a similar heat flux level, the ratio of dry to wetted area was determined to exceed 1/3, significantly higher than that predicted by a recent hydrodynamic model for CHF (approximately 1/16). This result suggests that modifications are needed for the hydrodynamic model when applied to highly wetting fluid on nearly isothermal surfaces. The merging of bubbles to form vapor blankets over the heater surface was observed, as has been assumed in recent hydrodynamic models.

Estimating daytime latent heat flux and evapotranspiration in Jamaica

The suitability of the two-parameter scheme of De Bruin and Holtslag (1982) to estimate daytime latent heat flux and evapotranspiration in Jamaica was examined using meteorological data collected over fields of short grass. The study covered three different periods, January to February of 1994, January to February of 1990, and August to September of 1989. The fields were irrigated regularly (three times a week) to provide enough water to evaporate. The daytime (07:00–18:00 hours) 20 min and hourly latent heat flux values, average evaporation for the earlier mentioned three periods and evaporation on individual days were estimated using De Bruin and Holtslag scheme for two situations. One was with the parameter α′=0.95 and β=20 W m −2 and the other was with α′=1.05 and β=20 W m −2. The analysis of the latent heat flux and evaporation led to the following results. (i) The values of latent heat flux and evaporation estimated using α′=0.95 and β=20 W m −2 compare well with the corresponding measured values for time steps of the order of 20 min to several days. (ii) The level of agreement between estimated and measured hourly values of latent heat flux seen in our work was very close to that reported in the study of De Bruin and Holtslag, implying the consistency of the skill of that scheme at higher average latent heat flux levels in Jamaica. (iii) The choice of 1.05 for α′ applies better for our data. These results indicate that the scheme of De Bruin and Holtslag with α′=0.95 and β=20 W m −2 can satisfactorily be used to estimate daytime latent heat flux and evaporation from well watered grass surfaces in regions having environmental conditions similar to those encountered in Jamaica, but appears better with α′=1.05.