Direct Heat Flux Research Articles

Thermal slip boundary conditions in vibrational nonequilibrium flows

This paper deals with new thermal jump boundary conditions at the wall in polyatomic gas flows. The calculations and results especially concern vibrational nonequilibrium conditions when the dissociation effects may be neglected. First the usual conditions are briefly described. Then the boundary conditions proposed in the paper are justified on the basis of previous experimental results and by developing direct heat flux calculations at the wall. Analytical expressions and numerical values of the various fluxes are presented.

A non-water-cooled heat flux measurement system under concentrated solar radiation conditions

This paper presents a brief description of a direct heat flux measurement system to measure the concentrated solar power delivered by a heliostat field onto the flat aperture of solar central receiver prototypes. The main advantages of this device are the low measurement uncertainty and the non-requirement of water-cooling. This system has been designed, mounted and used successfully on top of the SSPS-CRS tower at the Plataforma Solar de Almerı́a (PSA). The concentrated solar power and the spatial heat flux distribution, onto the aperture of a volumetric receiver prototype under evaluation, have been determined by the above-mentioned system.

Development and application of a novel technique for direct heat flux measurements in turbomachinery flows

The high performance and efficiency of modern gas turbines are only possible with temperatures inside the engine exceeding the allowed material temperatures in some areas by several hundred degrees. Therefore effective cooling methods are one of the key factors for the success of these engines. In order to achieve reliable predictions of the heat load of rotor or stator blades numerous research activities were performed to understand the nature of heat transfer in complex unsteady flows. Even numerical methods have made significant progress in recent years detailed experimental data are still necessary for validation and further development of the engines and the design tools. Here a new method to directly measure the heat flux $${\dot q}$$ at the material surface and accurately determine the heat transfer coefficienth is presented. The new sensor is based on the anisotropic characteristics of single crystals and allows the determination of the time varying heat flux on the surface of a model turbine airfoil. This feature is of special interest to study the influence of periodically disturbed flow conditions on the heat transfer characteristics of cooled turbine blades. The working principle of an anisotropic heat flux (AHF) sensor is briefly described together with the design of the actual sensor used in this study. Prior to the application of the sensor in a cascade test rig, comprehensive test of the sensor, the electronics and the data acquisition system were performed using a pulsed laser beam as heat source. To test the sensor under realistic conditions a large number of sensor was installed in a test blade and heat transfer measurements were performed in a cascade test rig equipped with a spoke-wheel wake generator. The results showed good agreement in the time mean results compared with standard techniques. Additionally time resolved data could be extracted from the sensor signals providing detailed information on the unsteady heat transfer characteristics and boundary layer development.

The Effect of Sea Spray Evaporation on Tropical Cyclone Boundary Layer Structure and Intensity*

Strong winds in a tropical cyclone over the ocean can produce high seas with substantial amounts of spray in the lower part of the atmospheric boundary layer. The effects that the evaporation of this sea spray may have on the transfer of energy between the ocean and the atmosphere, and consequent effects on the boundary layer structure, cumulus convection, and the evolution of the tropical cyclone, are largely unknown. In this study, a high-resolution tropical cyclone model with explicit cloud microphysics, developed by Y. Wang, has been used to study these potential effects. The sea spray evaporation is incorporated into the model by two bulk parameterization schemes with quite different properties. The numerical results show that inclusion of the Fairall et al. sea spray parameterization increases the direct sensible heat flux from the ocean by about 70%, but has little effect on the direct latent heat flux. Sea spray itself causes a sensible heat flux of only about 6% of the direct sensible heat flux, while it contributes a latent heat flux by evaporation of sea spray droplets by 60%‐70% of the direct latent heat flux. As a result, the total enthalpy flux with sea spray evaporation increases by about 20%, while the net contribution by sea spray is only about 1.5% of the total enthalpy flux. Consistent with this, the intensity of the model tropical cyclone is moderately increased by 8% in the maximum wind speed by the introduction of sea spray. The lower atmosphere becomes cooler and moister due to the evaporation of sea spray, which is supported by the available observations. The cooling in the surface layer further modifies the boundary layer structure and the activity of convection, especially in the near-core region where the highest concentration of sea spray exists. On the other hand, with the Andreas and DeCosmo parameterization scheme, the intensity of the model tropical cyclone is increased by 25% in maximum wind speed. This dramatic increase in the model tropical cyclone intensity is due to both the large net sensible heat flux and the latent heat flux associated with the effect of sea spray by this parameterization scheme. The net upward sensible heat flux warms the air near the surface and results in a near-isothermal surface layer in the near-core environment under the tropical cyclone. Such a structure, however, is not supported by the available observations, which the authors argue is not physically realistic. The radically different results with this scheme are due to the unusual way that the feedbacks between direct and spray-mediated fluxes are handled within the parameterization.

Regional Flux Estimation in a Convective Boundary Layer Using a Conservation Approach

Abstract During the Southern Great Plains 1997 Hydrology Experiment, a tethersonde system was deployed at the U.S. Department of Energy’s Atmospheric Radiation Measurement Cloud and Radiation Test Bed (ARM CART) central facility. Additional measurements included several surface flux stations at the central facility and radiosondes at the ARM CART central and boundary facilities. Combined, these data support an examination of regional flux estimates obtained via the atmospheric boundary layer conservation approach. Because the tethersonde was deployed successfully only under light to moderate wind conditions, the effects of advection on estimation of regional fluxes generally are found to be small. Consistent with previous studies, direct estimation of the sensible heat flux yields more accuracy than direct estimation of the latent heat flux. Use of available energy measured at surface flux stations along with the direct sensible heat flux estimates yields latent heat estimates of similar accuracy to those...

Influence of Surface Roughness on Heat Transfer and Effectiveness for a Fully Film Cooled Nozzle Guide Vane Measured by Wide Band Liquid Crystals and Direct Heat Flux Gages

The influence of surface roughness on heat transfer coefficient and cooling effectiveness for a fully film cooled three-dimensional nozzle guide vane (NGV) has been measured in a transonic annular cascade using wide band liquid crystal and direct heat flux gages (DHFGs). The liquid crystal methods were used for rough surface measurements and the DHFGs were used for the smooth surfaces. The measurements have been made at engine representative Mach and Reynolds numbers and inlet free-stream turbulence intensity. The aerodynamic and thermodynamic characteristics of the coolant flow have been modeled to represent engine conditions by using a heavy “foreign gas” (30.2 percent SF6 and 69.8 percent Ar by weight). Two cooling geometries (cylindrical and fan-shaped holes) have been tested. The strategies of obtaining accurate heat transfer data using a variety of transient heat transfer measurement techniques under the extreme conditions of transonic flow and high heat transfer coefficient are presented. The surfaces of interest are coated with wide-band thermochromic liquid crystals, which cover the range of NGV surface temperature variation encountered in the test. The liquid crystal has a natural peak-to-peak roughness height of 25 μm creating a transitionally rough surface on the NGV. The time variation of color is processed to give distributions of both heat transfer coefficient and film cooling effectiveness over the NGV surface. The NGV was first instrumented with the DHFGs and smooth surface tests preformed. Subsequently the surface was coated with liquid crystals for the rough surface tests. The DHFGs were then employed as the means of calibrating the liquid crystal layer. The roughness of 25 μm, which is the typical order of roughness for the in-service turbine blades and vanes, increases the heat transfer coefficient by up to 50 percent over the smooth surface level. The film cooling effectiveness is influenced less by the roughness. [S0889-504X(00)00804-7]

Remote True Temperature Pyrometry of Si Wafers: Theoretical and Practical Considerations

ABSTRACTThe problems of real thermodynamic temperature measurement in semiconductor processing can be divided into fundamental and environmental. The first type arises from a-priori unknown and in process changing optical properties (emissivity) of the target wafer. Others have to do with the need to eliminate direct and indirect stray heat flux from entering into the sensor. We describe the advantages and pitfalls of various (passive and active) remote measurement methods. Experimental results of a novel emissivity independent technique for real time measurement in the range of 450–850°C with accuracy better than 1% for wafer emissivity in the range of 0.2 to 0.95 are given.

Russian DEMO plant study

A conceptual design study of the DEMO reactor is being carried out in Russia. The main efforts are concentrated on the study of steady state reactor DEMO-S. This reactor will be operated with an advanced plasma regime involving a high fraction of bootstrap current. A divertor concept with open liquid lithium as a plasma facing material was chosen. Two extreme approaches were analyzed: the first assumes that liquid lithium is loaded with direct heat flux from 100 to 150 MW m −2; the consumption of liquid lithium and a form of lithium surface are defined; in the second approach lithium plasma dynamics in the divertor is analysed using the 2-D MHD divertor code. With this approach heat loads are considerably decreased by the radiation. Some additional issues connected with utilization of vanadium alloys in the DEMO reactor, including environmental and safety aspects, material activation and refabrication were also analyzed.

Power balance in DIII-D during single-null ELMing H-mode plasmas

We account for ≥85% of the injected power in DIII-D during single-null diverted ELMing H-mode discharges. Core plasma radiation accounts for ≤18% with the rest flowing through the scrape-off layer to the inboard and outboard divertors. The total power roughly splits in an in: out ratio of 1:1.3. Of the power entering the outboard divertor 75% leaves as direct heat flux to the divertor target plates with the rest in the form of radiation. In the inboard divertor, however, 75% of the power leaves as radiation with the rest as divertor target plate heat flux and charge exchange losses. The in: out ratio remains constant over a range of injected power and plasma current and even as divertor conditions change due to gas puffing.

A relationship between heat transfer to sea ice and temperature‐salinity properties of Arctic ocean waters

It is demonstrated that the characteristic temperature‐salinity relationship shown by thermocline waters of the Arctic Ocean can be reproduced using a simple model based on transfer of heat directly from these waters to sea ice. From this we deduce that there is direct sensible heat flux from waters of Atlantic origin to the ice cover, such interaction could play a major role in determining the physical properties of Arctic water masses.

Dynamics and thermodynamics of the ice/upper ocean system in the marginal ice zone of the Greenland Sea

Measurements were made of the ice/upper ocean system in the marginal ice zone of the Greenland Sea during a 4‐day storm in which wind blew the ice south across a frontal region, coinciding with the ice edge that had existed when the storm began. In response to wind stress of about 0.2 Pa, a turbulent boundary layer developed under the ice that exhibited marked Ekman rotation in both mean velocity (average surface speed about 0.17 m s−1, deflection angle about 33°) and turbulent stress profiles (typical Reynolds stress about 0.1 Pa). Ablation of the ice undersurface increased rapidly after crossing the surface temperature front, and the observed melt rate corresponded with direct heat flux measurements in the oceanic boundary layer, with maximum upward heat flux of about 200 W m−2. We discuss overall momentum and energy balances, interpret observed boundary layer measurements with a numerical model, and show that molecular effects are important for heat and mass transport at the hydraulically rough ice‐ocean interface. We also develop a simple model for predicting ice melt from interfacial stress and temperature and salinity of the mixed layer.

Superficial shell insulation in resting and exercising men in cold water.

From measurements of subcutaneous fat temperature (Tsf) at known depths below the surface, skin surface temperature (Tsk), and direct skin heat flux (H), the superficial shell isulation (Iss) of the thigh (fat + skin) was calculated as Iss (degrees C.m2.w-1) = (Tsf - Tsk)/H in nine male subjects immersed head out in a well-stirred water bath. Also, at critical water temperature (CWT = 28-33 degrees C), eight of the subjects rested for 3 h, enabling overall maximal tissue insulation (It,max) to be calculated as It,max (degrees C.m2.W-1) = (Tre - Tw)/(0.92 M +/- delta S), where Tre is rectal temperature, Tw is water temperature, M is metabolic rate, and s is loss or gain of body heat. Five subjects performed up to 2 h of mild leg cycling, preceded and followed by 60 min of rest, and both thigh Iss and overall It were measured during exercise. Iss increased from minimal values in Tw greater than 33 degrees C to maximal values (Iss,max) at CWT or below. Iss,max was linearly related to tissue thickness (d) in millimeters of fat plus skin, Iss,max (degrees C.m2.W-1) = 0.0048d-0.0052; r = 0.95, n = 37, and was not influenced by leg exercise up to a metabolic rate of 150 W.m-2 in CWT despite large increases in Tsf and H and large decreases in overall It. The slope of Iss,max vs. depth, 0.0048 degrees C.m2.W-1.mm-1, is almost identical to thermal resistivity of fat in vitro, suggesting that the superficial shell is unperfused in CWT at rest or during mild exercise. When maximal superficial shell insulation (It,ss,max) for the whole body was calculated with allowance for differing fat thicknesses and surface areas of body regions, it could account for only 10-15% of overall It,max at rest and 35-40% of overall It in mild exercise. We suggest that the poorly perfused muscle shell plays a more important role as a defense against cooling at CWT than does the superficial shell (fat + skin), particularly at rest.

OIL MIGRATION AND MODIFICATION PROCESSES IN SOLID SEA ICE

ABSTRACT The migration of Prudhoe Bay crude oil and diesel fuel through first year sea ice and the effect of entrained oil on sea ice growth rates were examined. Physical and chemical changes within the oils were examined during and after the entrainment process. Several crude oil and diesel fuel injections beneath solid sea ice were conducted off Prudhoe Bay during the winter of 1979/1980. Oil layers of 2.5, 15, and 30 centimeters (cm) were formed under 15, 30, and approximately 60 cm of sea ice. The sea ice growth under the injected oils was monitored. At the end of the ice growing season the individual oil injection sites were excavated from the ice. Each site was studied to determine the extent of vertical oil migration. The field results are compared to laboratory salt water ice growth experiments with entrained oil layers. The laboratory test results include oil migration as a function of ice temperature and the effect of various oil layer thicknesses upon the ice growth rates. Direct heat flux measurements are included from which the thermal conductivity of oil layers is obtained as a function of the oil layer thickness. These data illustrate the relationships among oil layer thickness, temperature gradient across the oil layer, oil viscosity, and convective motion occurring within the oil layer.

Flux Measurements, Flux Estimation Techniques, and Fine-Scale Turbulence Measurements in the Unstable Surface Layer Over Land

Abstract An AFCRL-UCSD joint experiment in Minnesota in 1973 has provided a comparison of direct and indirect measurements of the surface-layer fluxes of momentum, heat and moisture under unstable conditions. The direct momentum and heat flux measurements of the two groups agreed well, and also agreed well with values inferred by the direct dissipation technique. The moisture flux estimates from the inertial-dissipation technique also agreed well with the directly measured values. Several of the important terms in the budgets of turbulent kinetic energy and turbulent scalar variances were evaluated directly. The imbalance (or pressure transport) term in the energy budget was estimated, and the ratio of the imbalance term to the dissipation term determined from the present experiment agrees well with the Kansas results. The dissipation rate of temperature variance exceeded its production rate, in contrast with the Kansas results, implying an imbalanced temperature variance budget. Several possible contribu...

Application of the Reynolds Flux Model of Critical Heat Flux for Direct Heat Flux Scaling in Nonuniform Rod Bundles

Application of the Reynolds Flux Model of Critical Heat Flux for Direct Heat Flux Scaling in Nonuniform Rod Bundles

Refrigerant–Water Scaling of Critical Heat Flux in Round Tubes—Subcooled Forced-Convection Boiling

Subcooled, critical heat-flux test data on refrigerant R-11 flowing vertically in a uniformly heated circular tube is reported. Using the data reported by Coeffield for R-113, a method is presented for obtaining a direct heat-flux scaling factor between refrigerant and water with the same geometry, mass velocity, and local flowing quality when the pressure is adjusted so that the liquid-to-vapor density ratios are equal in both systems.