Maximum Value Of Heat Flux Research Articles

Numerical simulations of the solar transmission process for a pressurized volumetric receiver

A three-dimensional optical model for a pressurized volumetric receiver (PVR) is developed and corresponding solar radiation propagation process within the PVR is simulated by the Monte Carlo Ray Tracing (MCRT) method. In the computation, the complicated photon transmission process in the SiC porous absorber is simplified as the transmission process in the statistically homogeneous and isotropic turbid medium. Meanwhile, the non-uniform cylindrical coordinate grid is applied in the statistics of energy distribution, which could greatly reduce the number of cells in the computational grid and time compared with normal uniform grid. Based on the above model, the energy distribution in the irregular macro scale porous absorber is determined and then the effects of system parameters, including the incidence angle, the shape of absorber and the optical property of absorber, on the local heat flux of the absorber are investigated. The results show that, under the given operating condition, the radiation heat flux is mostly concentrated at the top area of the absorber and the maximum heat flux value is up to 2.73 × 109 W m−3, but it quickly decreases in the sideward locations. The incidence angle and a relative narrow shape of absorber are helpful to reduce the maximum heat flux in the absorber. Furthermore, as the ratio of absorption coefficient/extinction coefficient decreases, the absorbed radiation energy distribution is more uniform and the max heat flux in the absorber decreases greatly.

Evolution of flame to surface heat flux during upward flame spread on poly(methyl methacrylate)

The heat feedback profile across 5cm wide and 15cm tall samples of poly(methyl methacrylate) was studied from ignition until total sample involvement as a flame spread vertically upward. Incident heat flux to a water-cooled gauge was measured at 1cm intervals. At 6–15cm above the bottom edge of the flame, the maximum heat flux value was found to be on the order of 35kWm−2. Lower in the sample, 2–5cm above the flame bottom, where the flame is thinner and thus closer to the sample’s surface, the maximum heat flux is slightly higher, about 40kWm−2. Using these results and finely resolved measurements of sample burning rate recorded throughout the length of experiments, an analytical model that accurately predicts the measured heat flux profile along the vertical dimension of samples solely as a function of the burning rate was developed. Coupling this model with an accurate pyrolysis solver, which predicts material burning rate based on incident heat flux, is expected to enable highly accurate simulations of the flame spread dynamics.

Dimensional, physical and thermal comfort properties of plain knitted fabrics made from modal viscose yarns having microfibers and conventional fibers

The dimensional, some physical and thermal comfort properties of the plain knitted fabrics having modal viscose microfibers in three different stitch lengths are investigated in comparison with the similar fabrics having conventional modal viscose fibers. The fabrics made from microfibers and conventional fibers exhibit different dimensional properties. The stitch density results and the dimensional constants calculated at the fully relaxed state reveal that the fabrics with microfibers tend to have lower shrinkage tendency than those with conventional fibers. The statistical results show that the fiber type (or fiber fineness) and the stitch length affect the some physical properties and all of the thermal comfort properties of the fabrics significantly. The bursting strength values of the fabrics with microfibers are observed to be slightly higher than those of the fabrics with conventional fibers. However, the difference between the bursting strength values of these fabrics is found to be statistically unimportant. The fabrics with microfibers reveal lower thickness and air permeability and, higher pilling tendency than those with conventional fibers. It is also observed from the thermal comfort results that the fabrics made from microfibers have higher thermal conductivity, thermal absorptivity and maximum heat flux values and, lower thermal resistance and thermal diffusivity values. Because of the higher thermal absorptivity and maximum heat flux values, the fabrics from microfibers provide cooler feeling when compared with those from conventional fibers.

G060024 ヒートパイプによる成形金型の加熱・冷却システムの研究([G06002]熱工学部門一般セッション(2):ヒートパイプ・熱サイホン)

In this study, a heat pipe system was installed into the mold used in the polymer processing to enhance the heating and cooling rate of the mold as well as to reduce the required thermal energy for the temperature control of the mold, and the characteristics of the system were experimentally investigated. The result showed that the maximum heat flux value of 18 kW/m2 was achieved at the individual operation. By using the system, thermal energy can be transported between a pair of the molds in the sense of thermal district. The result showed that the maximum heat flux value of 8.0 kW/m2 was realized under the thermal energy exchange. And it was expected that the energy consumption to control the mold temperature would be reduced.

A MCRT and FVM coupled simulation method for energy conversion process in parabolic trough solar collector

A coupled simulation method based on Monte Carlo Ray Trace (MCRT) and Finite Volume Method (FVM) is established to solve the complex coupled heat transfer problem of radiation, heat conduction and convection in parabolic trough solar collector system. A coupled grid checking method is established to guarantee the consistency between the two methods and the validations to the coupled simulation model were performed. Firstly, the heat flux distribution on the collector tube surface was investigated to validate the MCRT method. The heat flux distribution curve could be divided into 4 parts: shadow effect area, heat flux increasing area, heat flux reducing area and direct radiation area. The heat flux distribution on the outer surface of absorber tube was heterogeneous in circle direction but uniform in axial direction. Then, the heat transfer and fluid flow performance in the LS-2 Solar Collector tube was investigated to validate the coupled simulation model. The outlet temperatures of the absorber tube predicted by the coupled simulation model were compared with the experimental data. The absolute errors are in the range of 1.5–3.7 °C, and the average relative error is less than 2%, which demonstrates the reliability of the coupled method established in this paper. At last, the concentrating characteristics of the parabolic trough collectors (PTCs) were analyzed by the coupled method, the effects of different geometric concentration ratios (GCs) and different rim angles were examined. The results show the two variables affect the heat flux distribution. With GC increasing, the heat flux distributions become gentler, the angle span of reducing area become larger and the shadow effect of absorber tube become weaker. And with the rim angle rising, the maximum value of heat flux become lower, and the curve moves towards the direction φ = 90°. But the temperature rising only augments with GC increasing and the effect of rim angle on heat transfer process could be neglected, when it is larger than 15°. If the rim angle is small, such as θ rim = 15°, lots of rays are reflected by glass cover, and the temperature rising is much lower.

Evaluation of the Wetness of Pantiliners

This study makes an objective evaluation of the wetness of pantiliners based on their physical properties and a subjective evaluation of the wetness of commercially produced pantiliners, based on subjective hand evaluation tests and wear trials performed by female subjects. The heat, air, and water transport properties of the pantiliners were measured using the Kawabata Evaluation System for Fabric (KES-FB) system, while the subjective wetness of the samples was analyzed using physical parameters. The wetness of pantiliners could be predicted from the water transport property. Those pantiliners that were evaluated as wet showed a large maximum value of heat flux under the wet condition ( qmax-wet). The qmax-wet value is one of the key parameters for evaluating the wetness of hygiene products.

The Initial Maximum Value of Heat Flux (qmax) as an Objective Measure of Fabric Warm/Cool Feeling

The Initial Maximum Value of Heat Flux (qmax) as an Objective Measure of Fabric Warm/Cool Feeling

Development and tests of ammonia Miniature Loop Heat Pipes with cylindrical evaporators

Miniature Loop Heat Pipes (MLHPs) are an attractive object for development and investigation as quite a promising means for cooling powerful electronics operating in the temperature range from 50 to 100 °C. The paper generalizes and presents the results of development and tests of 15 different variants of ammonia MLHPs with cylindrical evaporators 5 and 6 mm in diameter, which have an active zone length of 20 mm and are equipped with titanium and nickel wicks. As a result of successive efforts aimed at increasing the MLHPs efficiency, it was possible to achieve values of the heat-transfer coefficient close to 162,000 W/m 2 °C at a value of the heat flux of about 100 × 10 4 W/m 2. A maximum heat flux value of about 135 × 10 4 W/m 2 was achieved at the heat-transfer coefficient equal approximately to 75,000 W/m 2 °C.

Evaluation of "Shittori" Characteristic for Fabrics

“Shittori” in Japanese word of the human tactile sensation was not the same as smoothness. We evaluated the “shittori” feeling for fabrics by the sensory evaluation such as Kendall’s rank method, Scheffe’s method of paired comparison (modified method by Nakaya), and analyzed the relationship between the results by sensory evaluation and the mechanical, surface and thermal properties of fabrics measured by the KES system.The stronger “shittori” sensation for fabrics was accompanied with either the warm or soft. The yarn count and yarn density were related to the intensity of “shittori”. The effective characteristic values to understand “shittori” feeling of fabrics were qmax (the maximum value of heat flux), MIU (coefficient of friction), 2HB (hysteresis of bending moment), WC (compression energy) and thickness. In the cosmetics field, “shittori” expresses such state as the moisture is maintained in the skin, however in the case of “shittori” for fabrics, moisture regain of fabrics did not affect the subjective evaluation of “shittori”.

Change in Physical Properties of Cotton Fabric Caused by Natural Indigo Dyeing

Physical properties of cotton fabric, which was dyed with natural indigo reduced by fermentation, were studied. Cotton fabric showed maximum value of tensile energy, WT, tensile strength and abrasion resistance at dye uptake of 44 mg/g. Mean of friction coefficient, MIU, showed a maximum at dye uptake of 11.6 mg/g and then decreased with an increase in dye uptake. XRD reflection proved that cotton fiber was dyed with natural indigo in aggregate state. Natural indigo dyeing gave cotton fabric smaller air permeability, smaller heat conductivity and smaller initial maximum value of heat flux than synthetic indigo dyeing with an increase in dyeing times. The cotton fabric dyed with natural indigo exhibited larger mechanical properties except for abrasion resistance than those with synthetic indigo reduced by hydrosulfite at dye uptake of about 45 mg/g. It was suggested that natural indigo dyeing changed the structure of cotton fiber in association with high-order structure of fiber.

Experimental Investigation of CHF during Countercurrent Flow in Transient Boiling Systems

The objectives of this study are to investigate the effects of the power transient on critical heat flux (CHF), and to develop CHF and maximum heat flux (MHF) correlations in transient boiling systems with a countercurrent flow between the liquid and vapor flow. The test section consists of narrow, vertical, rectangular channels between parallel plates. Rate of change of wall temperature at the CHF point, (dTw/dt)CHF, and a nondimensional transient parameter, kappao =(L2/alphafTsat) (dTw/dt) CHF are introduced to evaluate the effects of power transients on CHF and MHF. Experimental ranges were 738.0-1,968.0 kg/m2s for mass flux, 7.0-17.5 C for inlet subcooling, and 3.0-8.0 mm for channel gap distance. The system pressure was kept constant at 1.0 atm. The experimental results show that CHF and MHF values increase with an increasing rate of change of wall temperature, and the increasing rate of CHF is higher in a wider channel gap distance. CHF and MHF increase linearly with increasing mass flux at the top of the test section, and the linearity decreases at the bottom of the test section. The effect of the inlet subcooling is significant at the top of the test section, therefore the test location closest to liquid inlet has a higher CHF value than the location immediately below it. It was also found that the wider the channel gap distance, the higher was the CHF value obtained for a given rate of change of wall temperature. New CHF and MHF correlations are developed for countercurrent flow in transient boiling systems. The CHF and MHF correlations are in good agreement with the experimental data within 25% error bands, respectively.

Transient boiling heat transfer from initial steady state caused by rapid depressurization

Transient boiling on single horizontal cylinders in water caused by rapid depressurization from initial natural convection or partial nucleate boiling was investigated. For an initial heat flux and a pressure reduction period respectively larger and shorter than certain threshold values, a transition to film boiling was observed. In a rapid transient due to depressurization from initial natural convection, the transient boiling heat transfer process followed a hypothetical nucleate boiling curve at the higher superheat side of the successive steady state nucleate boiling curves corresponding to each transient pressure. The maximum heat flux value was far lower than the steady state maximum heat flux at the corresponding pressure. The effects of various parameters, namely initial heat flux, pressure reduction rate, initial pressure and cylinder diameter, on the transient heat transfer processes (nucleate boiling heat transfer, maximum heat flux, transition to film boiling or not, film boiling heat transfer) were examined. A mechanism based on heterogeneous spontaneous nucleation from the originally flooded cavities on the solid surface was suggested for the transition from non-boiling to film boiling under rapid depressurization.

AN EXPERIMENTAL STUDY OF POST-CHF HEAT TRANSFER IN A 3x3 ROD BUNDLE

This chapter describes post-critical heat flux (CHF) heat transfer experiments in a 3×3 rod bundle. The objective of these experiments was to obtain measurements of thermodynamic nonequilibrium in the post-CHF regime and to characterize its effects on two-phase heat transfer. In boiling heat transfer systems a maximum value of heat flux called critical heat flux occurs when the heated surface can no longer support continuous liquid contact. A two-phase heat transfer loop was designed and built specifically for convective boiling heat transfer studies using steam-water mixtures. Each fuel rod simulator was equipped with 12 thermocouples such that two thermocouples were in the lower extension, one thermocouple in the upper extension and the remaining nine were in the test length. The data acquisition system consisted of a HP series 200, 9816A computer, HP 3497A Data Acquisition/Control unit and an HP34S6A Digital Voltmeter with all the devices on a single HP-IB interface bus, and a Bascom-Turner fast multichannel data recorder.

Temperature Mapping of Localized Hot Spots on Microelectronic Chip Surfaces

Abstract This paper describes the use of digital image processing in conjunction with an infrared imaging apparatus to locate and quantify “micro” hot spot temperatures on the surface of energized microelectronic chips. Briefly, the temperature mapping/processing procedure creates emissivity maps for the surface of the chip at different isothermal conditions. The emissivity map images are digitized and stored as a 512 × 512 pixel array, of which 400 lines contain IR information. Apparent temperature measurements are then collected with the chip energized in its normal operating environment. These apparent temperature data are digitized and stored as a 512 × 512 integer array using the same format as the digitized emissivity data. Before correcting for emissivity variations, the apparent temperature images are rectified using digital image processing to precisely overlay the spatial coordinates of the emissivity map. Finally, actual temperature maps are obtained by correcting the apparent temperature data for the local emissivity variations and background reflections. The computer driven measurement technique has been applied to the task of measuring localized temperatures on areas as small as 30 μm on the surface of an energized chip to an accuracy of ±1°C once the surface emissivity is accurately known. The infrared equipment, image processing hardware and supporting software are used to measure the temperature distribution on the surface of a 4.7 mm × 4.7 mm energized chip. IR measured temperatures at isolated locations on the chip are compared with results obtained by the resistance-temperature technique. Since the resistance-temperature technique provides an area-averaged temperature for the energized region, the result obtained from the high resolution IR measurements yields higher localized temperatures. Results are presented for peak surface temperatures up to 100°C and maximum heat flux values of 7.9x106 W/m2. A separate set of infrared measurements are used to predict the influence of surface emissivity on the accuracy of the temperature measurements.

Transient boiling caused by vapor film collapse at minimum heat flux in film boiling

This paper deals with the transient boiling heat transfer caused by vapor film collapse at minimum heat flux in film boiling for a horizontal rod heater in a pool of water under pressures from 100 to 400 kPa. Platinum rods of 2, 3, and 4 mm in diameter are used as the heater. In the transient boiling after the vapor film collapse, the heat flux increases at first from the minimum film boiling heat flux with rapid decrease in heater surface superheat. It arrives at a maximum heat flux value, and then decreases to nearly the same value as the minimum film boiling heat flux, where the heat transfer on the heater becomes again stationary in nucleate boiling or natural convection. The transient maximum heat flux is about 80% of the steady-state maximum heat flux of nucleate boiling independent of the pressure and heater diameter. The heater surface superheat at transient maximum heat flux is higher than the heater surface superheat at steady-state maximum heat flux. After the maximum heat flux is reached, the transient nucleate boiling heat flux approaches the steady-state nucleate boiling curve on heat-flux-versus-superheat graph and coincides with the curve in the lower heat flux region.

A numerical study of the effects of heat diffusion through the base of the mixed layer

A simple numerical model is used to investigate the warming of the mixed layer during the early summer. The structure consists of two layers separated by a thermocline, across which there is both downward diffusion of heat and upward entrainment of the lower layer by the warmer upper layer. Assuming that the diffusion rate in the thermocline varies with Richardson number (Ri) according to Munk and Anderson's (1948) formulation, it follows that the heat flux through the thermocline is small for either very small or very large gradients of temperature (i.e., density). The heat flux is maximum at a tempeature gradient corresponding to a ‘limiting Richardson number’ Ri* = 0.6. This maximum value of heat flux is proportional to the vertical velocity shear squared. Model results for a shallow area show that, for low surface heat fluxes throughout the early summer, weak warming occurs uniformly at all depths in proportion to the heat flux. For high surface heat fluxes, much stronger warming occurs, but it is confined to the surface layer. As a consequence the early summer warming of the cold bottom water actually will be decreased if the surface heat flux is increased beyond the maximum heat flux in the thermocline. If a 6‐day period of strong surface heating were to occur in the early spring, the model would predict that the resulting strong thermocline would persist for the remainder of the summer, allowing only 27% of the entire spring surface heat flux to penetrate to the lower layer. If the 6‐day period of strong surface heating were postponed until the end of the early summer, 52% more heat would be able to penetrate the thermocline during the entire spring. It is concluded that conditions favorable for the formation of a cold pool can prevail if the early summer surface heating is stronger or if a brief strong heating event occurs soon after the spring surface heating begins.