Decrease In Heat Transfer Coefficient Research Articles

Pressure swing adsorption modeling of acetone and toluene on activated carbon

A five steps pressure swing adsorption process was designed for acetone and toluene mixtures separation and recovery. Dynamic distributions of gas phase content and temperature were investigated. Based on the theory of Soret and Dufour, a non-isothermal mathematical model was developed to simulate the PSA process. Effects of heat and mass transfer coefficients were studied. The coupled Soret and Dufour effects were also evaluated. It is found that the heat transfer coefficient has little effect on mass transfer in adsorption stage. However, it has some impacts in desorption stage. The maximum value of C/C0 increases by about 25% as heat transfer coefficient decreases. The temperature variation is less than 0.05 K with the change of mass transfer coefficient, so that the effect of mass transfer coefficient on heat transfer can be ignored. It is also concluded that the Soret and Dufour coupled effects are not obvious in pressure swing adsorption compared with fixed-bed adsorption.

Conditions for thermal stabilization of the superconductor’s critical state

Conditions for thermal stabilization of the electrodynamic states of a superconductor are studied. The macroscopic states are simulated in the nonisothermal approximation by numerically solving a set of the Fourier and Maxwell equations with the magnetic flux penetration boundary unknown. Stability criteria for the critical state described by the viscous flow model are formulated. The results are compared with those fol� lowing from the isothermal theory. It is shown that errors inherent in the isothermal approximation are sig� nificant for a thermally insulated superconductor. Therefore, the wellknow n adiabatic criterion of stability formulated in the isothermal approximation limits the domain of stable states, since a correct determination of conditions for the superconducting-normal state transition must take into account the thermal history of the stable superconducting state formation. On the whole, the error of loss calculation in the isothermal approximation increases when the heat transfer coefficient decreases or an external magnetic field sweep and the size of the superconductor's cross section increases. On the other hand, nonisothermal stability condi� tions expand the variety of allowable states, since they include conditions that links the currently developed theory of thermomagnetic instability, the theory of losses, and the theory of a superconductor's thermal sta�

Comparative Evaluation of Hot-Air and Superheated-Steam Impinging Stream Drying as Novel Alternatives for Paddy Drying

An investigation was conducted on impinging stream drying of moist paddy using hot air and superheated steam as the drying media. Drying experiments were divided into two parts: namely, one-pass and two-pass drying. The volumetric water evaporation rate, volumetric heat transfer coefficient, and specific energy consumption of the drying system at various conditions were assessed; in the case of superheated-steam drying, the effect of steam recycle was also assessed. The quality of dried paddy was evaluated in terms of color, head rice yield, and degree of starch gelatinization. In the case of one-pass drying, an increase in the drying temperature led to a significant increase in the volumetric water evaporation rate and volumetric heat transfer coefficient. On the other hand, in the case of two-pass drying, an increase in the drying temperature led to a significant decrease in the volumetric heat transfer coefficient; the volumetric water evaporation rate was not significantly affected, however. The specific energy consumption decreased with an increase in the drying temperature. At the same temperature, using superheated steam as the drying medium led to lower specific energy consumption; higher level of steam recycle also led to more energy conservation. The color of the dried paddy was not affected by the change in the drying temperature; superheated-steam-dried paddy was redder and more yellow than the hot-air-dried paddy. An increase in the drying temperature led to decreased percentage of head rice yield. Superheated-steam drying helped enhance the level of starch gelatinization in comparison with hot-air drying at the same temperature. Nevertheless, drying at the highest tested temperature led to a lower level of starch gelatinization.

Investigation of two phase heat transfer and pressure drop of propane in a vertical circular minichannel

This article reports the flow boiling heat transfer and pressure drop results of propane in a vertical circular stainless steel minichannel having an internal diameter of 1.70mm and a heated length of 245mm. Two phase heat transfer and pressure drop experiments have been performed at saturation temperatures of 23, 33 and 43°C. Heat flux is varied from 5 to 280kW/m2 and mass flux is varied from 100 to 500kg/m2s. The results show that the two phase frictional pressure drops, as expected, are increased with the increase of mass flux, vapour qualities and with the decrease of saturation temperature. The heat transfer coefficients are showed to increase with the increase of heat flux and saturation temperature while the influence of mass flux and vapour quality is observed as insignificant. After incipience of dryout, the decrease in heat transfer coefficient and also the two phase frictional pressure drop, especially at higher mass fluxes, is observed. The two phase frictional pressure drop correlations of Müller-Steinhagen and Heck and Friedel and two phase flow heat transfer correlations of Cooper and Liu and Winterton well predicted the experimental results.

Thermoexcel-E 촉진 표면에서 임계 열유속까지의 풀 비등 열전달계수

In this work, nucleate pool boiling heat transfer coefficients(HTCs) of 5 refrigerants of different vapor pressure are measured on horizontal Thermoexcel-E square surface of 9.53 mm length. Tested refrigerants are R32, R22, R134a, R152a and R245fa. HTCs are taken from 10 <TEX>$kW/m^2$</TEX> to critical heat fluxes for all refrigerant at <TEX>$7^{\circ}C$</TEX>. Wall and fluid temperatures are measured directly by thermocouples located underneath the test surface and in the liquid pool. Test results show that critical heat fluxes(CHFs) of Thermoexcel-E enhanced surface are greatly improved as compared to that of a plain surface in all tested refrigerants. CHFs of all refrigerants on the Thermoexcel-E surface are increased up to 100% as compared to that of the plain surface. The improvement of Thermoexcel-E surface in CHF, however, is lower than that of the low fin surface. HTCs on Thermoexcel-E surface increase with heat flux. But after certain heat flux, HTCs began to decrease due to the difficulty in bubble removal caused by the inherent complex nature of this surface. Therefore, at heat fluxes close to the critical one, sudden decrease in HTCs needs to be considered in thermal design with Thermoexcel-E surface.

Correlative thermal methodology for castability simulation of ductile iron in ADI production

The present paper investigates the simulation analysis of simultaneous mold filling and solidification of ductile iron casting in a permanent mold by virtue of its thermal characteristics. Thermal analysis was performed to determine the solidification behavior and nature of alloy of the melt during its solidification. It revealed the variation in the nature of alloy due to the variations in eutectic freezing and carbon equivalent of the melt. The obtained thermal parameters from the thermal analysis were further used for the casting simulation of the melt. The simulation results show a progressive solidification behavior of the casting. There is a significant decrease in the overall heat transfer coefficient with time during the solidification process. The simulation results were further verified experimentally. The experimental results show porosity defects at the top section of the casting. Two distinct zones (center and outer) were observed on the produced samples based on the average graphite nodule counts and average graphite nodule size.

Heat transfer in a swirling fluidized bed with geldart type-D particles

A relatively new variant in fluidized bed technology, designated as the swirling fluidized bed (SFB), was investigated for its heat transfer characteristics when operating with Geldart type D particles. Unlike conventional fluidized beds, the SFB imparts secondary swirling motion to the bed to enhance lateral mixing. Despite its excellent hydrodynamics, its heat transfer characteristics have not been reported in the published literature. Hence, two different sizes of spherical PVC particles (2.61 mm and 3.65 mm) with the presence of a center body in the bed have been studied at different velocities of the fluidizing gas. The wall-to-bed heat transfer coefficients were measured by affixing a thin constantan foil heater on the bed wall. Thermocouples located at different heights on the foil show a decrease in the wall heat transfer coefficient with bed height. It was seen that only a discrete particle model which accounts for the conduction between the particle and the heat transfer surface and the gas-convective augmentation can adequately represent the mechanism of heat transfer in the swirling fluidized bed.

THERMAL TRANSPORT BEHAVIOR OF A LIQUID PLUG MOVING INSIDE A DRY CAPILLARY TUBE

Computational simulation of flow and heat -transfer characteristics of a single isolated liquid plug (of glycerin and water, respectively) flowing in a round capillary tube (internal diameter, D = 2.0 mm), so as to understand its local thermo-hy- drodynamic transport behavior, is reported. Both, constant heat flux and constant-tem - perature boundary conditions have been considered at the tube wall. When an isolated liquid plug moves in a capillary tube, there is a difference in the advancing and receding dynamic contact angles of the two menisci, respectively. This has been considered in the simulations. The linearized simplification of Hoffman-Tanner's law is used to model the variation in the two respective apparent dynamic contact angles with the velocity of the liquid plug (i.e., Ca = µU/σ). Simulations are carried out for a range of Capillary numbers and lengths of liquid plugs (L/D ratio). It has been found that variation in dy- namic contact angle leads to an enhanced local and average heat-transfer coefficient in the moving liquid plug; the local fluid circulation being affected by menisci deformation. In addition, as the length of the liquid plug is increased, the heat-transfer coefficient decreases and finally shows the asymptotic transport behavior of Poiseuille flow. Other than the flow Capillary number, the fluid Prandtl number and the L/D ratio of the liquid plug, strongly affect the local thermo hydrodynamics.

In-tube condensation heat transfer of CO2 at low temperatures in a horizontal smooth tube

This paper presents the experimental investigation of CO2 condensation in a horizontal smooth tube at saturation temperatures between 0 and −15°C with mass fluxes between 50 and 200kgm−2s−1 and for various vapour qualities. One tube-in-tube counter flow heat exchanger was designed together with an open-loop CO2 cycle to make up an experimental rig that condenses CO2 at these conditions. The inner test tube, with internal diameter of 6.52mm, is cooled using Glycol Water flowing in the outer tube. Experimental trends show that the heat transfer coefficient decreases with increasing saturation temperatures. Decrease in heat transfer coefficients is also observed for lower mass fluxes and lower vapour qualities in two-phase. Based on the experimental data collected, a new empirical correlation was developed to improve CO2 heat transfer coefficient prediction. The new correlation agreed well with experimental and data from the literature, with an average deviation of −4.4%.

INFLUENCE OF SURFACE COATING WITH XANTHAN GUM ON HEAT TRANSFER DURING DEEP‐FAT FRYING OF POTATO STRIPS

ABSTRACTPotato strips were coated with xanthan gum solutions (0.3, 0.6, 0.9%, w/v) which exhibited shear‐thinning and weak gel‐like characteristics. Then, the effects of the surface coating on heat transfer during deep‐fat frying of the potato strips were investigated as a function of concentration of xanthan gum solutions. Deep‐fat frying of surface‐coated potato strips exhibited sigmoidal temperature profiles which were shown to be dependent on the concentration of xanthan gum solutions. Heat transfer coefficients were also determined to be between 215 and 254 W/m2 K. The use of higher concentration of xanthan gum led to a distinct decrease in heat transfer coefficients during frying of potato strips, which were shown to be highly correlated to the viscosity of xanthan gum solution (R2 = 0.99). It thus suggested that the surface coating of potato strips with xanthan gum might create an insulating layer, consequently reducing the rate of heat transfer during frying.PRACTICAL APPLICATIONSCoating and batters in the frying industry are the common methods to reduce the oil uptake of fried foods. However, few experiments have been carried out to investigate the changes in the heat transfer during frying when a food matrix is coated or battered. In this study, a food matrix coated with hydrocolloid solutions was subjected to deep‐fat frying during which heat transfer coefficients were investigated. Our results obtained in this study can provide a fundamental understanding for the mechanisms of heat transport in coated (or battered) fried food systems, consequently helping optimization and control of a frying process.

Thermal characterization of shrouded plate fin array on an LED backlight panel

This study experimentally investigates a 348 mm × 558 mm LED backlight panel consisting of 270 1-W LEDs and a plate-fin heat sink in an acrylic housing. Effects of shroud clearance, obstructions at entrance or exit on the overall performance are also examined. The results show that the heat transfer coefficient is very slightly reduced with the rise of shroud clearance from 0 mm to 5 mm, followed by a notable rise but it peaks at a shroud clearance between 10 and 20 mm. A further increase of shroud clearance leads a marginal decrease of heat transfer coefficient. It is found that the maximum temperatures within the LED panel are not necessarily located at the center region due to the edge effect. On the other hand, the temperature variation along the longitudinal direction indicates a local minimum Nusselt number occurring near the exit of LED panel. This unique phenomenon is associated with the development of velocity and temperature field, and is consistent with previous numerical examinations. On the other hand, the obstruction placed at the exit are often severe than at the entrance.

Study on Surface Heat Transfer Coefficient of Mixture of Nitrogen-Spray Water Eject Quenching under Normal Pressure and High Velocity

During mixture of Nitrogen and spray water ejecting quenching under normal pressure and high velocity, the liquid film that is formed on the surface of specimen to reduce the heat transfer between specimen and quenching media is removed on the one hand; on the other hand, the heat transfer performance of the mixture exceeds that of pure Nitrogen. Because the surface heat transfer coefficient is difficult to measure, according the cooling curve of surface and centre of specimen measured experimentally, the law of surface heat transfer coefficient and specimen temperature is calculated by nonlinear estimate methods and finite difference method based on inverse heat transfer method. The results show that the cooling performance of mixture of Nitrogen and spray water is as well as that of water or oil. During quenching, the surface heat transfer coefficient increases rapidly at begin, and at temperature of 170 °C, the surface heat transfer coefficient decreases. During martensite phase transformation, the latent heat is used to increase drive force of phase transformation and to overcome resistance of phase transformation, thus the martensite phase transformation can fulfill.

Performance Experimental Study and Energy Consumption Calculation of Hollow Glass Window Film

This paper taked hollow glass window as the model, and heat transfer coefficient, sunshading coefficient and energy consumption of the window glass were obtained in the situations of window glass with and without low-E film by doing experiment and theoretical calculation. The results show that both heat transfer coefficient and sunshading coefficient decrease comparing the hollow glass window film with no film. For the fixed and push-pull window of the whole window, the heat transfer coefficient decrease by 24.2% and 28.9% respectively and sunshade coefficient decreases by 31.37%. And the low-E film by this study adopting reduces energy consumption of the summer greatly by 32.96%, and has no too large effect on winter heating, so the annual energy consumption reduces and the window film saves annual energy consumption 14.62% .

HETRA experiment for investigation of heat removal from the first wall of helium-cooled-pebble-bed test blanket module

This paper presents an experimental test section which has been developed at the Institute for Neutron Physics and Reactor Technique in Karlsruhe Institute of Technology for investigations of heat removal from the first wall (FW) of the Helium-Cooled-Pebble-Bed Test Blanket Module (HCPB TBM). The test section named HETRA is attached to HEBLO facility which can provide Helium parameters relevant for HCPB TBM conditions–pressure of 8 MPa and inlet temperature of 300 °C. The purpose of planned investigations is to verify numerically found decrease of heat transfer coefficient due to a pronounced asymmetry of heat loads at the first wall of HCPB TBM. This paper describes how HETRA test section has been built: from its design over fabrication to the assembling. The planned measurements are also explained by presentation of a detailed measuring scheme. Finally, the first results concerning experimental and computational determination of pressure losses in HETRA cooling channel are presented.

Investigation on the characteristics and mechanisms of unusual heat transfer of supercritical pressure water in vertically-upward tubes

The heat transfer characteristics of supercritical pressure water in a vertically-upward optimized internally-ribbed tube was investigated experimentally to study the mechanisms of unusual heat transfer of supercritical pressure water in the so-called large specific heat region. The experimental parameters were as follows. The pressure at the inlet of the test section ranged from 22.5 to 29.0 MPa, and the mass flux of the fluid was from 650 to 1200 kg/m 2 s, and the heat flux on the inside wall of the tube varied from 200 to 660 kW/m 2. According to experimental data, the characteristics of heat transfer enhancement and also the heat transfer deterioration of supercritical pressure water in the large specific heat region was analyzed and based on the comparison and analysis of the current major theories that were used to explain the reasons for unusual heat transfer to occur, the mechanisms of heat transfer enhancement and deterioration were discussed, respectively. The enhanced heat transfer was characterized by the gently changing wall temperature, the small temperature difference between the inside-tube-wall and the bulk fluid and the high heat transfer coefficient in comparison to the normal heat transfer. The deteriorated heat transfer could be characterized by the sharply increasing wall temperature, the large temperature difference and a sudden decrease in heat transfer coefficient in comparison to the normal heat transfer. The heat transfer enhancement of the supercritical pressure water in the large specific heat region was suggested to be a result of combined effect caused by the rapid variations of thermophysical properties of the supercritical pressure water in the large specific heat region, and the same was true of the heat transfer deterioration. The drastic changes in thermophysical properties near the pseudocritical points, especially the sudden rise in the specific heat of water at supercritical pressures, might result in the occurrence of the heat transfer enhancement, while the covering of the heat transfer surface by fluids lighter and hotter than the bulk fluid made the heat transfer deteriorated eventually and explained how this lighter fluid layer formed.

Experimental study on condensation heat transfer of steam on vertical titanium plates with different surface energies

Visual experiments were employed to investigate heat transfer characteristics of steam on vertical titanium plates with/without surface modifications for different surface energies. Stable dropwise condensation and filmwise condensation were achieved on two surface modification titanium plates, respectively. Dropwise and rivulet filmwise co-existing condensation form of steam was observed on unmodified titanium surfaces. With increase in the surface subcooling, the ratio of area ( η) covered by drops decreased and departure diameter of droplets increased, resulting in a decrease in condensation heat transfer coefficient. Condensation heat transfer coefficient decreased sharply with the values of η decreasing when the fraction of the surface area covered by drops was greater than that covered by rivulets. Otherwise, the value of η had little effect on the heat transfer performance. Based on the experimental phenomena observed, the heat flux through the surface was proposed to express as the sum of the heat flux through the dropwise region and rivulet filmwise region. The heat flux through the whole surface was the weighted mean value of the two regions mentioned above. The model presented explains the gradual change of heat transfer coefficient for transition condensation with the ratio of area covered by drops. The simulation results agreed well with the present experimental data when the subcooling temperature is lower than 10 °C.

Laminar heat transfer of non-Newtonian nanofluids in a circular tube

Forced convection heat transfer behavior of three different types of nanofluids flowing through a uniformly heated horizontal tube under laminar regime has been investigated experimentally. Nanofluids were made by dispersion of γ-Al2O3, CuO, and TiO2 nanoparticles in an aqueous solution of carboxymethyl cellulose (CMC). All nanofluids as well as the base fluid exhibit shear-thinning behavior. Results of heat transfer experiments indicate that both average and the local heat transfer coefficients of nanofluids are larger than that of the base fluid. The enhancement of heat transfer coefficient increases by increasing nanoparticle loading. At a given Peclet number and nanoparticle concentration the local heat transfer coefficient decreases by axial distance from the test section inlet. It seems that the thermal entry length of nanofluids is greater than the base fluid and becomes longer as nanoparticle concentration increases.

Effect of salt spray corrosion on air-side hydrophilicity and thermal-hydraulic performance of copper-fin heat exchangers

The effect of salt spray corrosion on the air-side hydrophilicity and the thermal-hydraulic performance of copper-fin heat exchangers were experimentally investigated. Artificial accelerated method of salt spray corrosion on the copper-fin heat exchangers was used for simulating the actual corroded heat exchangers. The experimental results show that, the contact angles increase with the increase of salt spray corrosion hours, which results in the degradation of the hydrophilicity of copper fin. The air-side heat transfer coefficients decrease and pressure drops increase with the increase of corrosion hours. The effect of salt spray corrosion on the heat transfer coefficients and pressure drops become more obvious with the increase of inlet air velocity. The heat transfer coefficients of the corroded copper-fin heat exchangers decrease by 4.4–34.0% and the pressure drop increase by 5.2–26.1% comparing with those of the uncorroded copper-fin heat exchanger at the inlet air velocity ranging from 0.5 to 2.0 m/s.

Numerical modeling and analysis of the thermal behavior of copper molds in continuous casting

In this study, we establish a 3-D numerical analysis model to analyze the thermal behavior and to obtain the detailed heat transfer coefficient of a copper mold in a continuous casting system. This heat transfer coefficient changes according to variations in the mold geometry or cooling system. For increased flow speeds of the cooling water, the heat transfer coefficient also increases, but the rate of increase for the coefficient diminishes at higher flow speeds. As the thickness of the mold between the melt and the cooling water slots increases, the mold’s heat transfer coefficient decreases. However, the uniformity of the heat transfer coefficient improves with greater thickness. The effect of distance between cooling water slots on the mold’s heat transfer coefficient is also observed. Calculations show that greater distances between cooling water slots decrease the heat transfer coefficient.

G213 油中水滴型エマルジョンの熱伝達に対するアルコール添加の影響(一般講演)

Experimental study has been conducted to examine the heat transfer characteristics of fuel and water emulsion. Water in fuel emulsion with alcohol additives were prepared with base fuel, distilled water and regent grade of alcohol with small amount of surfactant. Heat transfer experiments were carried out on an electrically heated horizontal thin Pt wire. It is concluded that the heat transfer coefficient decrease with an increase of ethanol contents. This tends to be similar to the case of methanol additive in the emulsions. Effects on heat transfer of ambient emulsion temperature was also revealed.