Average Heat Flux Values Research Articles

Influence of hot water pipe position on the performance of radiant floor heating systems integrated with PCM

To address the slow heating and low efficiency issues of traditional macro-packed phase change material (PCM) radiant floor heating systems, this study innovatively explores the impact of altering the eccentric positions of hot water pipes within the PCM encasement, based on computational fluid dynamics (CFD) numerical simulations. Starting with a concentric case as a baseline, the investigation extends to six different configurations achieved by displacing the hot water pipes vertically (upward and downward) and horizontally. The analysis encompasses the PCM's thermal state, temperature dynamics, spatial distribution, and heat flux. The results indicate that the arrangement of hot water pipes with upward and horizontal displacements demonstrates a more positive impact on improving the thermal performance of the PCM radiant floor heating system, with the effect becoming more pronounced as the displacement distance increases. Cases with upward eccentric movement of r/4 (where r is the encasement radius) and r/2 reached the optimal floor temperature of 297.15 K faster than the concentric case by 2 h and 3.8 h, respectively, during the heating phase. Cases with horizontal eccentric movement advanced by 0.5 h and 2.5 h, respectively. Cases with horizontal displacements of r/4 and r/2 caused smaller temperature fluctuations and provided greater comfort and energy efficiency than those with upward displacements, with average heat flux values being 4.75 % and 11.82 % lower, respectively. These findings offer theoretical support for the optimization and application of macro-packed PCM floor heating systems.

Heat transfer performance analysis of front-end capillary heat exchanger of a subway source heat pump system

During the operation of subway systems, a significant amount of heat is stored annually in the rock surrounding the tunnel, which results in underground thermal pollution. A subway source heat pump system with a front-end capillary heat exchanger (CHE) is considered an effective technology to address this problem. However, it has not yet been employed in systematic design and operation methods. This study numerically simulates and analyses the heat transfer performance of a tunnel lining CHE for an entire year under the typical conditions of an actual project. The results are expected to provide theoretical guidance for CHE design and operation. Moreover, the proposed model was verified by comparing the simulation results with those of field experiments.The simulation results indicated that both the temperature and heat flux of the CHE and tunnel surrounding rock experienced periodic fluctuations during the heating and cooling seasons and exhibited a strong heat recovery ability in the transitional seasons. The temperature of the surrounding rock (Tmid) gradually decreased with increasing depth of the surrounding rock, and the change in Tmid at greater depths (≥15 m) was less than 0.1 and 0.13 °C during the heating and cooling seasons, respectively. However, phase delays and amplitude attenuation phenomena occurred during the harmonic temperature transfer process. The phase delay of the tunnel wall temperature (Tw) valley and peak during the heating and cooling seasons were 13 and 2.2 h from the CHE wall temperature (TC) valley and peak, respectively. It was suggested that the efficient operation of the CHE should harmonise the two temperature waves of the tunnel wall and CHE. Furthermore, the average heat flux values at the lining-CHE outer wall contact surface during the CHE operation were −16 and 22 W during the heating and cooling seasons, respectively. In addition, the CHE exhibited abnormal heat fluxes during the heating and cooling seasons along the CHE pipe (10 m), indicating that the design and operation of the CHEs must consider the influence of the pipe length.

DETERMINATION OF CONVECTIVE, RADIATIVE, AND TOTAL HEAT TRANSFER CHARACTERISTICS OVER A RADIANT HEATED CEILING: A COMPUTATIONAL APPROACH

also been given in international standards for experimental approaches. In accordance with the experimental standards, the analyses are realized at the room dimensions of (L x H x W= 4 x 3 x 4 m), and at the thermal boundary conditions of (Tc= 35 – 50oC), (Tw= 15 – 24oC), (Tf= 15 – 24oC). As a result of iterations conducted using a computational fluid dynamics package program code, each corresponding to a case study, convective, radiative, and total heat fluxes from radiant heated ceiling are obtained, and afterward using relevant reference temperature proposed in the literature, heat transfer coefficients pertaining to convection, radiation, and total heat transfer have been found. In conclusion, average heat flux values of 95.83 Wm-2 and 17.49 Wm-2 are gained through the examined radiant heated ceiling, by means of radiation and convection, respectively. This denotes that roughly 85% of total heat transfer from the ceiling arises through radiation that is very compatible with the relevant literature. Finally, spanning the entire different case studies, average convective, radiative, and total heat transfer coefficients of 1.1 Wm-2K-1, 5.4 Wm-2K-1, 7.6 Wm-2K-1 are acquired.

Heat-flux enhancement response for novel flow-boiler operations under resonant, sub-harmonic, and superharmonic imposition of vapor pulsation frequencies relative to a liquid flow-rate pulsation frequency

This paper presents fundamental results from experimental investigations of shear/pressure-driven internal flow boiling of FC-72 in a horizontal test section of total flow channel length 1000 mm, gap height 2 mm, and width 15 mm. It compares effects of the frequencies of vapor and liquid pressure fluctuations (pulsations) on the experimentally measured heat-flux rates and heat transfer coefficients at a representative location within the flow boiler under a flow operation method in which re-circulating vapor maintains an annular flow regime over the entire length of the device. For liquid flow/pressure pulsations at 3.8–3.9 Hz, four externally imposed inlet vapor pressure fluctuation conditions were considered: (1) no externally imposed vapor pulsations and (2)–(4): high-amplitude externally imposed vapor pulsations at (2) the same, (3) half, and (4) double the inlet liquid pulsation frequency. Representative pressure differences within the flow boiler are also examined. Local time-averaged heat-flux and heat transfer coefficient responses of these flows are compared with previously published data at the same liquid pulsation frequency, but at a different inlet liquid flow rate, vapor quality, and pressure. The reported measurements, and the discussions and conclusions in this paper, enable better understanding of an established pulsation-induced heat-flux enhancement phenomenon which may be used in future cooling systems to significantly enhance average heat-flux values over the entire length of an annular flow boiler. The main conclusion is that, for the flow conditions investigated, optimal heat transfer efficiency occurs when vapor and liquid pulsations are imposed at the same frequency on the flow-boiler inlet.

Sub-cavity liquid volume beneath spray droplet impacts into static liquid layers, and initial estimates of the heat flux required to dry out this volume

The time variation of the sub-cavity liquid volume beneath individual droplet impact cavities was measured for ranges of drop Weber and Reynolds numbers that match those for a full cone spray nozzle of interest. Cavity lifetime was also measured. These results will be used in a Monte-Carlo model of the spray cooling process that is currently under development. Droplet Weber numbers were varied between 140 and 1000. Corresponding Reynolds numbers ranged between approximately 1200 and 3600. These ranges matched Phase Doppler results that were obtained for the water spray under study. Thickness of the static liquid layer into which the single droplets impacted was varied between 0.2 and 1.0 times the droplet diameter.The measured sub-cavity liquid volume generally was between 60% and 80% of the original droplet volume over much of the cavity lifetime. Increasing the static liquid layer thickness increased this plateau value of the sub-cavity liquid volume between these lower and upper bounds, for Weber numbers greater than around 400. Sub-cavity liquid volume also increased somewhat as Weber number was increased. The cavity lifetime increased significantly as Weber number increased.The sub-cavity liquid volume and cavity lifetime results were scaled to values for the corresponding spray droplets at equal (We, Re). These were then used in an energy balance between the energy transferred through a heated wall to the sub-cavity liquid and the sum of the sensible heating and latent heat required to dry out the spray drop sub-cavity liquid volume within the cavity lifetime. These computed heat fluxes to dry out the drop impact cavities were then used to estimate the overall average heat flux for a heated surface that would dry out these individual spray droplet impact cavities. The resulting average heat flux values were between 400W/cm2 and 800W/cm2. These predictions are similar to the range of CHF values reported in the literature for water, of 500W/cm2 to 1000W/cm2.

Convective heat transfer equation for turbulent flow in tubes applied to internal combustion engines operated under motored conditions

An equation developed for the case of turbulent flow in tubes was applied for the study of convective heat transfer in internal combustion engines. Calculated average heat flux values under motored conditions were compared to measurements, results available in the literature obtained by using a computational fluid dynamics (CFD) code and the correlations of Woschni and Annand. Cumulative heat flux values for the four models were also compared to measurements at the end of the compression stroke and start of expansion. Average heat transfer rates were found to be predicted with reasonable accuracy by the proposed model. The major advantage of this equation compared to the correlations of Woschni and Annand is that it does not require any corrections for specific engine characteristics or working conditions. The proposed model was found to correctly predict higher heat transfer rates as engine speed, load and compression ratio were increased. Accuracy at the end of the compression stroke was found to be within acceptable limits for the cumulative heat flux, comparable to that of the CFD code, except for one case of the seven different engines investigated. Therefore, this model can be used to predict spatial average convective heat flux values with fairly good accuracy under motored conditions, when limited information on engine geometry is available, or when computing resources are required to be minimal.

Heat-Transfer Measurements in the Primary Cooling Phase of the Direct-Chill Casting Process

Thermal modeling of the direct-chill casting process requires accurate knowledge of (1) the different boundary conditions in the primary mold and secondary direct water-spray cooling regimes and (2) their variability with respect to process parameters. In this study, heat transfer in the primary cooling zone was investigated by using temperature measurements made with subsurface thermocouples in the mold as input to an inverse heat conduction algorithm. Laboratory-scale experiments were performed to investigate the primary cooling of AA3003 and AA4045 aluminum alloy ingots cast at speeds ranging between 1.58 and 2.10 mm/s. The average heat flux values were calculated for the steady-state phase of the casting process, and an effective heat-transfer coefficient for the global primary cooling process was derived that included convection at the mold surfaces and conduction through the mold wall. Effective heat-transfer coefficients were evaluated at different points along the mold height and compared with values from a previously derived computational fluid dynamics model of the direct-chill casting process that were based on predictions of the air gap thickness between the mold and ingot. The current experimental results closely matched the values previously predicted by the air gap models. The effective heat-transfer coefficient for primary cooling was also found to increase slightly with the casting speed and was higher near the mold top (up to 824 W/m2·K) where the molten aluminum first comes in contact with the mold than near the bottom (as low as 242 W/m2·K) where an air gap forms between the ingot and mold because of thermal contraction of the ingot. These results are consistent with previous studies.

Convective heat transfer studies in macro and mini channels using digital interferometry

Convective heat transfer in micro and mini channels has been recommended as an effective heat removal method for various electronic packages and systems. As the traditional method of temperature measurement in channels using the thermocouple probe disturbs the flow field leading to measurement errors, a non-intrusive measurement technique, such as an optical method is preferable for temperature and heat transfer measurement in mini channels. In the present work, convective heat transfer studies have been performed on water flowing through channels of small cross sections of hydraulic diameter 4 mm and 3 mm, using digital interferometric technique. The channels are fabricated using high quality optical glass and aluminum blocks. Mach–Zehnder Interferometry is used for obtaining the temperature distribution in the channels. The interferograms grabbed using a CCD camera, coupled to a computer and digital image processing technique, has been used for obtaining the temperature distribution from the fringes. The temperature profiles are obtained at different sections of the channels for various values of the average Reynolds number and various heating levels. The local and average heat flux values are obtained from the constructed temperature distributions. Results of parametric studies are compared and contrasted with relevant entry length solutions from the literature.

Enceladus: An estimate of heat flux and lithospheric thickness from flexurally supported topography

We have identified flexural uplift along a rift zone of the Harran Sulci, Enceladus, using Cassini images and stereo‐derived topography. On the assumption that the upraised topography is related to a flexed elastic plate, shape modeling yields an effective lithospheric thickness of 0.3 km and, combined with the strength envelope, a mechanical lithospheric thickness of 2.5 km with heat fluxes of 200–270 mW/m2 at the time of formation. The heat fluxes are comparable to average heat flux values measured in Enceladus' active south polar region, and they are consistent with estimates derived via models of unstable extension of the lithosphere at this location. Surface porosity can reduce the obtained heat fluxes to an estimated minimum of 45–60 mW/m2. Crater‐size frequency counts fix the time of formation (from present‐day) of the rift zone at either 3.5−0.4+0.1 Ga (assuming an asteroid‐type impact chronology) or 0.45−0.3+1.1 Ga (assuming a comet‐type impact chronology).

Field tests on human tolerance to (LNG) fire radiant heat exposure, and attenuation effects of clothing and other objects

A series of field tests exposing mannequins clothed with civilian clothing to a 3 m × 3 m square liquefied natural gas (LNG) pool fire was conducted. Both single layer clothing and double layer clothing were used. The radiant heat flux incident outside the clothing and incident on the skin covered by clothing were measured using wide-angle radiometers, for durations of 100–200 s (per test). The levels of heat flux incident on the clothing were close to 5 kW/m 2. The magnitude of the radiant heat attenuation factor (AF) across the thickness was determined. AF varies between 2 and higher for cotton and polyester clothing (thickness 0.286–1.347 mm); AF value of 6 was measured for 1.347 mm thickness. Single sheet newspaper held about 5 cm in front of mannequins and exposed to incident flux of 5 kW/m 2 resulted in AF of 5, and AF of 8 with double sheets. AF decreases linearly with increasing heat flux values and linearly increases with thickness. The author exposed himself, in normal civilian clothing (of full sleeve cotton/polyester shirt and jean pants), to radiant heat from a LNG fire. The exposure was for several tens of seconds to heat flux levels ranging from 3.5 kW/m 2 to 5 + kW/m 2 (exposure times from 25 s to 97 s at average heat flux values in the 4 kW/m 2 and 5 kW/m 2range). Occasionally, he was exposed to (as high as) 7 kW/m 2 for durations of several seconds. He did not suffer any unbearable or even severe pain nor did he experience blisters or burns or any other injury on the unprotected skin of his body. The incident heat fluxes on the author were measured by a hand-held radiometer (with digital display) as well as by strapped on wide-angle radiometers connected to a computer. He could withstand the US regulatory criterion of 5 kW/m 2 (for 30 s) without suffering any damage or burns. Temperature measured on author's skin covered by clothing did not rise above the normal body temperature even after 200 s of exposure to 4 kW/m 2 average heat flux.

The effect of the liquid motion induced by air and vapor bubbles on heat transfer around a cylinder

This paper reports on a study into the effects of bubble-induced motion on heat transfer around a cylinder. Local heat flux due to vapor or air injected bubble motion is measured at different positions around a heated copper cylinder using a hot film technique as well as a heat flux sensor. The first set of experiments, carried out during the first stages of nucleate boiling, showed a dependence of the heat transfer on surface orientation. A similar conclusion was reached in the second set of experiments where the air bubbles were injected using an external source. Average heat flux values computed in the two cases on the upper part of the cylinder were shown to compare relatively well. Finally, the mechanisms involved in the heat transfer enhancement at the same location, on the upper part of the cylinder, in the two sets of experiments are described using a simultaneous analysis of the heat flux time-series and bubble motion.

2708 水蒸気-空気混合気の細い鉛直管外面への自由対流層流膜状凝縮

An experimental study is presented on free convection laminar film condensation of steam-air mixture on a vertical tube. The test tube used is of about 220mm height and about 6mm outer diameter. The range of concentration of air is 0.1∿0.9. The average heat flux values or average heat transfer coefficients obtained have been about 2.5 times larger than the corresponding values from the two-phase boundary layer theory for a vertical flat surface. The enhanced values are quantitatively explained by the curvature effect upon the vapor boundary layer, which is analogically derived from single phase free convection heat transfer.

Laminar film condensation of steam containing small concentrations of air

Film condensation heat flux measurements have been made on a vertical surface at film Reynolds numbers below the critical values for ripple formation. These measurements have been made on air-steam mixtures with air mass fractions of 1 × 10 −4−1 × 10 −2 and on pure steam at saturated vapor temperatures of 150, 115 and 80°F. The average heat flux values for air-steam mixtures have been divided by those computed from the Nusselt theory for pure vapor at the same difference between vapor and surface temperature. The ratios so obtained are found to be about 20 per cent above the theoretical predictions of Minkowyez and Sparrow.