Minimum Heat Transfer Coefficient Research Articles

냉매 이성분 혼합물의 포화 풀핵비등 특성에 관한 연구

Saturated nucleate pool boiling experiments for binary mixtures, which are consisted of refrigerant R11 and R113, were performed with constant wall temperature condition. Results for binary mixtures were also compared with pure fluids. A microscale heater array and Wheatstone bridge circuits were used to maintain the constant temperature of the heating surface and to obtain heat flow rate measurements with high temporal and spatial resolutions. Bubble growth images were captured using a high speed CCD camera synchronized with the heat flow rate measurements. The departure time for binary mixtures was longer than that for pure fluids, and binary mixtures had a higher onset of nucleate boiling (ONB) temperature than pure fluids. In the asymptotic growth region, the bubble growth rate was proportional to a value between and . The bubble growth behavior was analyzed to permit comparisons with binary mixtures and pure fluids at the same scale using dimensionless parameters. There was no discernable difference in the bubble growth behavior between binary mixtures and pure fluids for a given ONB temperature. And the departure radius and time were well predicted within a error. The minimum heat transfer coefficient of binary mixtures occurred near the maximum value, and the average required heat flux during bubble growth did not depend on the mass fraction of R11 as more volatile component in binary mixtures. Finally, the results showed that for binary mixtures, a higher ONB temperature had the greatest effect on reducing the heat transfer coefficient.

Heat Transfer in Two-Pass Rotating Rectangular Channels (AR=1:2 and AR=1:4) With 45 Deg Angled Rib Turbulators

This paper reports the heat transfer coefficients in two-pass rotating rectangular channels (AR=1:2 and AR=1:4) with rib roughened walls. Rib turbulators are placed on the leading and trailing walls of the two-pass channel at an angle of 45 deg to the flow direction. Four Reynolds numbers are considered from 5000 to 40 000. The rotation numbers vary from 0.0 to 0.3. The ribs have a 1.59 by 1.59 mm square cross section. The rib height-to-hydraulic diameter ratios e/Dh are 0.094 and 0.078 for AR=1:2 and AR=1:4, respectively. The rib pitch-to-height ratio P/e is 10 for both cases, and the inlet coolant-to-wall density ratio (Δρ/ρ) is maintained around 0.115. For each channel, two channel orientations are studied, 90 deg and 45 deg with respect to the plane of rotation. The results show that the rotation effect increased the heat transfer on trailing wall in the first pass, but reduced the heat transfer on the leading wall. For AR=1:4, the minimum heat transfer coefficient was 25% of the stationary value. However, the rotation effect reduced the heat transfer difference between the leading and trailing walls in the second pass.

Single bubble growth in saturated pool boiling of binary mixtures

Nucleate pool boiling experiments for binary mixtures, which are consisted of R11 and R113, were performed with constant wall temperature condition. Results for binary mixtures were also compared with pure fluids. A microscale heater array and Wheatstone bridge circuits were used to maintain the constant temperature of the heating surface and to obtain heat flow rate measurements with high temporal and spatial resolutions. Bubble growth images were captured using a high-speed CCD camera synchronized with the heat flow rate measurements. The departure time for binary mixtures was longer than that for pure fluids, and binary mixtures had a higher onset of nucleate boiling (ONB) temperature than pure fluids. In the asymptotic growth region, the bubble growth rate was proportional to a value between t 1/6 and t 1/4. The bubble growth behavior was analyzed to permit comparisons with binary mixtures and pure fluids at the same scale using dimensionless parameters. There was no discernible difference in the bubble growth behavior between binary mixtures and pure fluids for a given ONB temperature. And the departure radius and time were well predicted within a ±30% error. The minimum heat transfer coefficient of binary mixtures occurred near the maximum | y− x| value, and the average required heat flux during bubble growth did not depend on the mass fraction of R11 as more volatile component in binary mixtures. Finally, the results showed that for binary mixtures, a higher ONB temperature had the greatest effect on reducing the heat transfer coefficient.

Heat transfer in nucleate pool boiling of multicomponent mixtures

Steady state pool boiling heat transfer coefficients have been obtained experimentally for acetone–isopropanol–water and acetone–MEK (methyl ethyl ketone)–water ternary systems. The data show that, for a given heat flux, the heat transfer coefficients of mixtures are lower than those obtained for pure components constituting the mixture. The measured heat transfer coefficients were compared with predictions from literature correlations for pool boiling of multicomponent mixtures. In all the cases, overestimation or underestimation of the data was observed. The literature correlations incorporate an `ideal' heat transfer coefficient and a correction term for the presence of other liquids. Part of the uncertainty associated with the literature correlations on comparison with the present data appears to be due to the uncertainty in estimating the ideal heat transfer coefficient. The methods suggested by the earlier authors found to be not applicable for the present heating surface–liquid combinations. Therefore, in the present study, two different correlations were tried for the ideal heat transfer coefficient and it was found that the performance of the literature correlations improved considerably. However, the methods reported in the literature and the correlations tried in the present study to estimate ideal heat transfer coefficient did not consider heating surface–liquid interaction and the effect of heating surface micro-roughness in boiling. Therefore, a correlation to estimate the ideal heat transfer coefficient has been proposed taking into account surface–liquid interaction parameter and surface roughness group. A new correlation to estimate mixture heat transfer coefficients has been proposed in the present study in terms of an ideal heat transfer coefficient and a correction term. In general, the correction term in binary mixtures is obtained by incorporating the binary diffusivity of the mixture. In multicomponent systems, the multicomponent diffusion coefficients have to be incorporated in the expression for the correction term. The mixture heat transfer coefficient correlation along with the correlation for ideal heat transfer coefficient proposed in the present study predicts the present experimental data as well as literature data satisfactorily. The heat transfer coefficient was found to be a function of the difference between the equilibrium vapor and liquid concentration of the light component(s) and the minimum heat transfer coefficient occurs at the maximum of this value.

PHENOTYPIC VARIATION OF LARKS ALONG AN ARIDITY GRADIENT: ARE DESERT BIRDS MORE FLEXIBLE?

We investigated interindividual variation and intra-individual phenotypic flexibility in basal metabolic rate (BMR), total evaporative water loss (TEWL), body temperature (Tb), the minimum dry heat transfer coefficient (h), and organ and muscle size of five species of larks geographically distributed along an aridity gradient. We exposed all species to constant environments of 15°C or 35°C, and examined to what extent interspecific differences in physiology can be attributed to acclimation. We tested the hypothesis that birds from deserts display larger intra-individual phenotypic flexibility and smaller interindividual variation than species from mesic areas. Larks from arid areas had lower BMR, TEWL, and h, but did not have internal organ sizes different from birds from mesic habitats. BMR of 15°C-acclimated birds was 18.0%, 29.1%, 12.2%, 25.3%, and 4.7% higher than of 35°C-acclimated Hoopoe Larks, Dunn's Larks, Spike-heeled Larks, Skylarks, and Woodlarks, respectively. TEWL of 15°C-acclimated Hoopoe Larks exceeded values for 35°C-acclimated individuals by 23% but did not differ between 15°C- and 35°C-acclimated individuals in the other species. The dry heat transfer coefficient was increased in 15°C-acclimated individuals of Skylarks and Dunn's Larks, but not in the other species. Body temperature was on average 0.4°C ± 0.15°C (mean ± 1 sem) lower in 15°C-acclimated individuals of all species. Increased food intake in 15°C-acclimated birds stimulated enlargement of intestine (26.9–38.6%), kidneys (9.8–24.4%), liver (16.5–27.2%), and stomach (22.0–31.6%). The pectoral muscle increased in 15°C-acclimated Spike-heeled Larks and Skylarks, remained unchanged in Hoopoe Larks, and decreased in 15°C-acclimated Woodlarks and Dunn's Larks. We conclude that the degree of intra-individual flexibility varied between physiological traits and among species, but that acclimation does not account for interspecific differences in BMR, TEWL, and h in larks. We found no general support for the hypothesis that species from desert environments display larger intra-individual phenotypic flexibility than those from mesic areas. The coefficient of variation of larks acclimated to their natural environment was smaller in species from arid areas than in species from mesic areas for mass-corrected BMR and surface-specific h, but not for mass-corrected TEWL. The high repeatabilities of BMR, TEWL, and h in several species indicated a within-individual consistency on which natural selection could operate.

Mixed Convection Transport From an Isolated Heat Source Module on a Horizontal Plate

An experimental study of the mixed convective heat transfer from an isolated source of finite thickness, located on a horizontal surface in an externally induced forced flow, has been carried out. This problem is of particular interest in the cooling of electronic components and also in the thermal transport associated with various manufacturing systems, such as ovens and furnaces. The temperature distribution in the flow as well as the surface temperature variation are studied in detail. The dependence of the heat transfer rate on the mixed convection parameter and on the thickness of the heated element or source, particularly in the vicinity of the source, is investigated. The results obtained indicate that the heat transfer rate and fluid flow characteristics vary strongly with the mixed convection variables. The transition from a natural convection dominated flow to a forced convection dominated flow is studied experimentally and the basic characteristics of the two regimes determined. This transition has a strong influence on the temperature of the surface and on the heat transfer rate. As expected, the forced convection dominated flow is seen to be significantly more effective in the cooling of a heat dissipating component than a natural convection dominated flow. The location of the maximum temperature on the module surface, which corresponds to the minimum local heat transfer coefficient, is determined and discussed in terms of the underlying physical mechanisms. The results obtained are also compared with these for an element of negligible thickness and the effect of a significant module thickness on the transport is determined. Several other important aspects of fundamental and applied interest are studied in this investigation.

Natural convection heat transfer in the separation region of a backward-facing step. (Heat transfer from step).

An experimental study has been conducted to investigate the fluid flow and heat transfer of natural convection past a backward-facing step. The step and the vertical plates upstream and downstream of the step are heated uniformly with a constant heat flux, and water is used as a test fluid. The flow and temperature fields over the step are visualized. The result shows that the high-temperature fluid ascending from the upstream plate separates from the step after flowing over a certain distance from the step edge, while the low-temperature fluid above the step rushes onto the step surface after the separation. The local heat transfer coefficients of the step are also measured, and the results reveal that the above separation and inrush of a high- and low-temperature fluid yield the minimum and maximum heat transfer coefficients of the step. It is also found that the locations of the minimum and maximum heat transfer coefficients are determined solely with the local Grashof numbers of the step.

The results of 5% small-break LOCA tests and natural circulation tests at the ROSA-IV LSTF

Five 5% small-break loss-of-coolant accident (SBLOCA) experiments and two natural circulation experiments were conducted at the ROSA-IV Large Scale Test Facility (LSTF). The liquid holdup in the upflow side of steam generator (SG) U-tubes temporarily depressed the core collapsed liquid level below the bottom of core during the loop seal clearing in the cold-leg break SBLOCA tests. This phenomena was affected by the core power and core bypass but was affected little by the actuation of the high pressure injection system. Overall thermal-hydraulic phenomena in a loop seal line break test was similar to that of cold-leg break tests, however, the liquid holdup phenomena played a little role. In a hot-leg break test a temporary but rapid depression of the core liquid level was observed immediately after the initiation of accumulator injection which caused condensation and depressurization in the cold leg. The change of natural circulation flow rate with the decrease of primary system mass inventory was qualitatively the same as observed in Semiscale, LOBI and PKL. The SG effective overall heat transfer coefficient below the secondary-side collapsed liquid level was weakly dependent on the secondary side liquid level and the core power. The measured minimum heat transfer coefficient was 1.7 kW/m 2K for the full secondary side mass inventory.

Some Characteristics of Heat Transfer Deterioration of Flow Boiling in a One-Side-Heated vertical Finned Smooth Bore Tube

This paper presents some results of heat transfer deterioration (HTD) tests that were conducted on a finned smooth bore tube of 11 mm nominal to evaluate the effect of one-sided heating on the HTD phenomenon in once-through power boiler tubes. The one-side radiantly heated test sections were operated using upward steam-water flow at pressures of 179, 189, and 193 atm. Using peak heat flux at the tube wall crown as the correlating parameter and comparing with uniform heating data, the effects of one-sided heating on HTD quality and minimum heat transfer coefficient were found.

Heat transfer between a vertical tube and an air fluidized bed at moderate temperatures (335–528 K)

Heat transfer coefficient is measured between a 25.4 mm outer diameter smooth vertical tube immersed in 0.254 m internal diameter fluidized beds of sand (d p = 265, 507 and 787 μm), glass beds (239, 357 and 412 μm), silicon (368 μm) and alumina (267 μm) as a function of bed temperature (335 – 528 K) and fluidizing velocity. The solids movement and mixing in the bed is investigated by measuring the heat transfer coefficient at two radial positions in the bed for various values of fluidization velocity and bed temperature. The minimum fluidization velocity, bed voidage and maximum heat transfer coefficient are also reported for different experimental conditions. The qualitative variation of h o with different operating and system parameters is found consistent with the known mechanisms of heat transfer for an immersed surface in a fluidized bed. Quantitative correlation of h o and h omax will be reported in a future publication.

Natural Convection Heat Transfer from Vertical Rectangular-Fin Arrays : Part 3, Heat Transfer from Fin-Flats

An experimental study has been made of natural convection heat transfer from the fin-flat, one of the component surfaces of a fin array ; and the effect of fin geometry and temperature on the heat transfer coefficient has been clarified. From these data and the results of the author's previous reports, an empirical formula for the average Nusselt number of vertical open-channel is derived. Furthermore, a comparison in heat transfer between open-channels and rectangular ducts has been carried out. The existence of the minimum heat transfer coefficient for the fin-length is qualitatively confirmed.

Heat Transfer from Electrically Heated Horizontal Wire to Sprayed Jet

The main object of the present investigation is to study heat transfer to sprayed jet by using of electrically heated wire. As the temperature difference between the heating surface and the water increases, heat transfer to boiling water occurs by several reimes-natural convection, nucleate boiling, transition boiling and film boiling. And the existence of maximum and minimum rates of evaporation which are recognized as Nukiyama point and Leidenfrost point is well known. Heat transfer to sprayed jet is quite similar in shape to the boiling curves, and the following empirical formulas are obtained for various values of diameter D mm of wires, diameter d of sprayed drops, volume ratio Vw/Va of water-air and Reynolds number Re, [numerical formula]and [numerical formula] where (Nu)n. and (Nu)l are Nusselt number at the points of maximum and minimum heat transfer coefficients, and (Δt)n and (Δt)l are the critical temperature difference (°C) corresponding to them.