Differences In Heat Transfer Characteristics Research Articles

Effect of Misalignment at the Flat and Profiled Endwall of Nozzle Guide Vane on Heat Transfer

Misalignment of gas turbine engines is caused by the difference in thermal expansion from the design point when starting or stopping the engine, or when operating at partial load. This study evaluated the differences in heat transfer characteristics between a flat endwall and a profiled endwall with misalignment. To ensure consistency with the flow conditions, experiments were conducted at an outlet Reynolds number of 300,000 for both geometries. The experiment was performed under three different conditions: a case without step, a case with a forward-facing step (h/Cx= -0.1), and a case with a backward-facing step (h/Cx= 0.1). In the flat endwall, the thermal load of the upstream region increases significantly with both step circumstances compared to the case without a step. The misalignment causes a recirculation flow to occur between the interface of the combustor and turbine. This leads to the mainstream reattaching upstream of the endwall. Nevertheless, the heat transfer characteristics of the profiled endwall differ from those of the flat endwall under step conditions. Most locations do not experience significant changes, with only a slight increase observed in the upstream region. The recirculation flow is crucially reduced due to the intensity of the secondary vortex in the profiled shape. Hence, the increase in the thermal load on the profiled endwall with step is considerably lower than that on the flat endwall. As a result, the thermal load on the profiled endwall has decreased by 25 % for the forward-facing step and 20.7 % for the backward-facing step compared to the flat endwall.

A novel constant pressure control method for nucleation boiling of CO2/lubricant in molecular dynamics simulation

Conventional molecular dynamics simulations of nucleation boiling typically employ the fixed volume method, resulting in a non-constant pressure during the boiling process. In this paper, we propose an additional pressure control method, placing a cover plate at the top of the liquid phase to achieve liquid phase pressure control during boiling. The novel method yields a pressure control error within ±5 %. CO2 and CO2/PEC4 lubricating oil mixture are used to investigate the differences in heat transfer characteristics between non-constant and constant pressure nucleation boiling. Furthermore, the effect of PEC4 lubricating oil on CO2 nucleation boiling under a fixed pressure condition is explored. The results show that both the bubble nucleation and growth time of CO2 and the CO2/PEC4 mixture are prolonged under constant pressure. This causes a compression in CO2 molecular spacing, thereby reducing CO2 diffusion and mass transfer capabilities. Heat transfer from the solid to the liquid, and within the liquid itself, is diminished, while the distribution of PEC4 molecules within the liquid phase becomes more even. The migration rate of PEC4 molecules in the liquid phase slows down. Under the constant pressure model, PEC4 increases the mass transfer resistance of CO2. As the lubricant mass fraction rises, The nucleation and growth rate of CO2 bubbles decline. In comparison to CO2, PEC4 molecules exhibit a stronger interaction energy with the Cu wall surface. During boiling, some PEC4 molecules accumulate on the Cu surface, forming an oil film that impedes the direct contact between CO2 molecules and the Cu wall.

Experimental study on convection heat transfer of supercritical CO2 in small upward channels

A supercritical carbon dioxide (s-CO2) Brayton power cycle was extensively applied in several energy systems in this study. This paper presents an experimental investigation of the heat transfer characteristics of supercritical CO2 in a vertical, small circular tube with an inner diameter of 5 mm at a low mass flux but high heat fluxes. Experiments were performed for inlet temperatures ranging from 5 to 40 °C, pressures from 7.5 to 10.5 MPa, mass flux from 50 to 200 kg/m2s, and heat flux from 38 to 234 kW/m2. Results reveal marked differences in heat transfer characteristics under different levels of heat flux at low mass fluxes in the small channel. Buoyancy plays a substantial role in heat transfer at low mass flux, especially under conditions with high heat fluxes. Based on data from this study and experimental results in small channels described in the literature, a new heat transfer model was developed using a deep learning method with the TensorFlow tool, which captures 90% of experimental data within a ±20% error band.

Experimental study of pure steam and steam–air condensation over a vertical corrugated tube

The natural convection condensation heat transfer outside the vertical smooth tube and corrugated tube was experimentally investigated in pure steam and steam–air conditions, respectively. The aim of the research was to evaluate the heat removal capacity of the corrugated tube through comparing with the test results of the smooth tube. In the experiment, a corrugated tube was tested, the bulk temperature and the wall temperature of crest and trough were measured in detail. The result of the temperature measurement demonstrated that the difference of heat transfer characteristics at the crest and trough is not significant. In the pure steam condition, the influence of subcooling on condensation heat transfer coefficient of the corrugated tube is reduced. Comparing with the smooth tube, the condensation heat transfer coefficient of the corrugated tube has been increased for more than 10%. In the steam–air test condition, the condensation heat transfer coefficients have been obtained for the air mass fraction from 0.1 to 0.95 and the total pressure from 0.2 MPa to 0.5 MPa. The corrugated tube shows a slight enhancement with an average increase of condensing heat transfer coefficient about 10%.

Comparison of Diesel Engine Efficiency and Combustion Characteristics Between Different Bore Engines

This study investigates the effects of engine bore size on diesel engine performance and combustion characteristics, including in-cylinder pressure, ignition delay, burn duration, and fuel conversion efficiency, using experiments between two diesel engines of different bore sizes. This study is part of a larger effort to discover how fuel property effects on combustion, engine efficiency, and emissions may change for differently sized engines. For this specific study, which is centered only on diagnosing the role of engine bore size on engine efficiency for a typical fuel, the engine and combustion characteristics are investigated at various injection timings between two differently sized engines. The two engines are nearly identical, except bore size, stroke length, and consequently displacement. Although most of this diagnosis is done with experimental results, a one-dimensional model is also used to calculate turbulence intensities with respect to geometric factors; these results help to explain observed differences in heat transfer characteristics of the two engines. The results are compared at the same brake mean effective pressure (BMEP) and show that engine bore size has a significant impact on the indicated efficiency. It is found that the larger bore engine has a higher indicated efficiency than the smaller displaced engine. Although the larger engine has higher turbulence intensities, longer burn durations, and higher exhaust temperature, the lower surface area to volume ratio and lower reaction temperature leads to lower heat losses to the cylinder walls. The difference in the heat loss to the cylinder walls between the two engines is found to increase with increasing engine load. In addition, due to the smaller volume-normalized friction loss, the larger sized engine also has higher mechanical efficiency. In the net, since the brake efficiency is a function of indicated efficiency and mechanical efficiency, the larger sized engine has higher brake efficiency with the difference in brake efficiency between the two engines increasing with increasing engine load. In the interest of efficiency, larger bore designs for a given displacement (i.e., shorter strokes or few number of cylinders) could be a means for future efficiency gains.

Research on Applicability of Hot-Bulb Anemometer under Low Pressure

In Mars and other deep space exploration missions, the planetary atmosphere makes the difference in heat transfer characteristics on the planetary surface and on-orbit environment. In order to achieve the purposes of thermal model correction and spacecraft verification in extreme environment, Mars rover needs to be tested in a simulated Mars environment including low pressure, solar heat flux, wind speed and background temperature. Thus, wind speed should be measured at multiple points in the Mars rover thermal test. In a general Mars rover thermal balance test, the requirement for wind speed control and measurement is 0-15m/s under 700Pa pressure. The current anemometer for industrial use is mainly based on the dynamic pressure, heat or ultrasound. They have a small signal and need to be recalibrated at low pressures. In this paper, a constant heat flux hot-bulb anemometer model has been built using dimensionless number analysis method, with which the anemometer response under low pressure has been calculated. A series of calibration test has been employed to verify the model in space environment chamber. The two methods above reached a similar result, which demonstrates the effectiveness of the analysis.

Experimental study on the difference of heat transfer characteristics between vertical and horizontal flows of supercritical pressure water

The present paper is devoted to investigating the difference of heat transfer characteristics between horizontal and vertical upward flows of supercritical pressure water. An Experimental study is conducted with both horizontal and vertical upward tubes (∅32mm×3mm), covering a range of mass fluxes (G) from 200 to 600kg·m−2·s−1, heat fluxes (q) up to 400kW·m−2, and pressure (P) from 23 to 28MPa. Heat transfer characteristics are analyzed in detail for selected parameters. The results show at low q/G, an apparent heat transfer enhancement and insignificant difference in the two arrangements. However, when the q/G increases to a higher value (i.e. q/G>0.5), heat transfer deterioration occurs and a noticeable heat transfer discrepancy is detected, where the inner-wall temperature of vertical flow far exceeds that of horizontal flow. Dimensionless parameters, Bo+, Kν, and BTH are adopted to analyze the effects of buoyancy force and thermal acceleration for both flows. The analysis suggests that mechanisms governing horizontal and vertical flows of supercritical pressure water are different at high q/G or in deteriorated heat transfer mode. For the vertical flow, thermal acceleration plays a leading role, while for the horizontal flow, the effect of buoyancy plays a larger effect than that for vertical flow.

Influence of buoyancy on heat transfer to water flowing in horizontal tubes under supercritical pressure

The present paper was devoted to clarifying the effect of buoyancy on heat transfer characteristics of supercritical pressure water flowing in horizontal tubes. Experiments were conducted in horizontal tubes with inner diameters of 26 mm and 43 mm. Operating conditions included mass fluxes of 300–1000 kg/m2 s, heat fluxes up to 400 kW/m2, and a pressure of 25 MPa. Based on the experimental data, the difference of heat transfer characteristics between the top and bottom surfaces of a horizontal tube was analyzed. It was found that the buoyancy effect makes the low density hot water gather at the top surface of the horizontal tube; hence, heat transfer condition is deteriorated and wall temperature is increased. The effects of mass flux, flow direction and tube diameters on the heat transfer characteristics of water in horizontal tubes were discussed in detail. Jackson and Petukhov buoyancy criterions were selected to judge the buoyancy effect in horizontal tubes. The results showed that when the buoyancy effect is significant, both criterions are able to predict the onset of buoyancy effect. Furthermore, numerical simulations were carried out by using Fluent solver, aiming to extending experimental research on buoyancy effect.

Experiment of Heat Transfer to Supercritical Water Flowing in Vertical Annular Channels

An experimental study of heat transfer to supercritical water has been performed at Xi'an Jiaotong University with a vertical annular tube. The annular test sections were constructed with an annular gap of 2 mm and an internal heater of 8 mm outer diameter. Experimental parameter covered pressures of 23 and 25 MPa, mass fluxes of 700 and 1000 kg/m2s, and heat fluxes of 200–1000 kW/m2. Experimental data were acquired from downward flow and upward flow, respectively. There were differences of heat-transfer characteristics between the two flow directions. Compared to upward flow, the heat-transfer coefficient increased at downward flow. A strong effect of spacer on heat transfer is observed at locations downstream of the device in the annuli regardless of flow direction. The spacer effect impaired the buoyancy effect at low heat flux, but not for large heat flux. Complex of forced convection and mixed convection in supercritical water is due to various thermophysical properties and the gravity. The affected zone of the spacer effect depends on the flow conditions. The buoyancy effect was analyzed qualitatively in this study and the criterion Gr¯/Re2.7<10-5 for negligible heat-transfer impairment was discussed. Four correlations were compared with the experimental data; the Swenson correlation predicted nearly the experimental data but overpredicted slightly the heat-transfer coefficients.

H125 冷媒HCFC123の細管内核沸騰の可視化 : 沸騰開始点近傍における気泡成長(OS-11:沸騰・二相流(2))

This paper presents an experimental investigation on forced convective boiling of refrigerant HCFC123 in a mini-tube, and visualization of boiling phenomena at the onset points of nucleate boiling is conducted. It was indicated in the former report that heat transfer characteristics of forced convective boiling in mini-tubes, of which inner diameter is φ0.51mm and φ0.30mm, were different from those in conventional tubes. In order to clarify this difference of heat transfer characteristics, bubble formation and growth at the onset points of nucleate boiling were observed and examined, using a glass tube on which ITO film is formed. For low heat flux, an isolated bubble grows up to occupy the whole tube cross section, and flows in the form such as a bullet. As heat flux becomes larger, plural bubbles grow to coalesce into a long vapor plug, and evaporation through liquid film between the vapor plug and the inner tube surface becomes dominant.

The inaccuracy of heat transfer characteristics for non-insulated and insulated spherical containers neglecting the influence of heat radiation

In this investigation, the differences of heat transfer characteristics for insulated and non-insulated spherical containers between considering and neglecting the influence of heat radiation are studied by the simulations in some practical situations. It is found that the heat radiation effect cannot be ignored in conditions of low ambient convection heat coefficients (such ambient air) and high surface emissivities, especially for the non-insulated and thin insulated cases. In most practical situations when ambient temperature is different from surroundings temperature and the emissivity of insulation surface is different from that of metal wall surface, neglecting heat radiation will result in inaccurate insulation effect and heat transfer errors even with very thick insulation. However, the insulation effect considering heat radiation will only increase a very small amount after some dimensionless insulated thickness (such insulation thickness/radius ≧ 0.2 in this study), thus such dimensionless insulated thickness can be used as the optimum thickness in practical applications. Meanwhile, wrapping a material with low surface emissivity (such as aluminum foil) around the oxidized metal wall or insulation layer (always with high surface emissivity) can achieve very good insulated effect for the non-insulated or thin insulated containers.

Experimental Investigation on Observed Scattering in Heat Transfer Characteristics for Flow Boiling in a Small Diameter Tube

The effects of flow fluctuation at the test section inlet on flow boiling heat transfer were investigated for FC72 by the use of a horizontal circular tube with a diameter of 0.51 mm. Flow fluctuation was minimized by employing a high-powered syringe pump in one experiment and intensified in another experiment by the connection of an auxiliary tank exposed to atmosphere allowing the reverse flow. In the experiments of strict inlet flow rate regulation, heat transfer characteristics were similar to those observed in normal size tubes. However, if the flow rate fluctuated by up to ± 20% of the total under the weak inlet flow rate regulation, the trend of the heat transfer coefficient increasing with increasing vapor quality in moderate vapor quality region disappeared, and heat transfer deterioration due to partial dryout started at lower vapor quality. Boiling heat transfer characteristics in minichannels could be changed considerably by the existence of flow fluctuation caused by the rapid axial growth of elongated bubbles at low vapor quality. The regulation of inlet flow rate seems to be a key parameter in reducing the scattering in heat transfer data encountered in flow boiling of minichannels. Experimental data obtained by using a pump of weak power or data using a liquid reservoir can never be inherently consistent with those obtained for the constant inlet flow rate conditions. Although such a weak restriction of inlet conditions is actually encountered in application systems, the difference in heat transfer characteristics between the normal and mini-tubes should be clarified, as the first objective of the research, under the same inlet conditions without flow fluctuation.

이산화탄소/프로판 혼합냉매의 수평평활관 및 마이크로 핀관에서의 증발열전달에 관한 실험적 연구

Abstract Evaporation heat transfer characteristics of CO 2 /propane mixtures in horizontal smooth and micro-fin tubes have been investigated by experiment. The experiments were carried out for several test conditions of mass fluxes, heat fluxes, compositions of CO 2 /propane refrigerant mixtures and tube geometries. Direct heating method was used for supplying heat to the refrigerant where the test tube was uniformly heated by electric current which was applied to the tube wall. Heat transfer coefficient data during evaporation process of CO 2 /propane mixtures were measured for 5 m long smooth and micro-fin tubes with outer diameters of 5 mm, respectively. The tests were conducted at mass fluxes of 318 to 997 kg/m 2 s, heat fluxes of 6 to 20 kW/m 2 and for several mixture compositions (100/0, 75/25, 50/50, 25/75, 100/0 by wt% of CO 2 /propane). The differences of heat transfer characteristics between smooth and micro-fin tubes for various compositions of CO 2 /propane refrigerant mixtures and the effect of mass flux, and heat flux on enhancement factor (EF) and penalty factor (PF) were presented.

Experimental study on the evaporative heat transfer and pressure drop of CO 2 flowing upward in vertical smooth and micro-fin tubes with the diameter of 5 mm

Because of the ozone layer depletion and global warming, new alternative refrigerants are being developed. In this study, evaporation heat transfer characteristic and pressure drop of carbon dioxide flowing upward in vertical smooth and micro-fin tubes were investigated by experiment with regard to evaporating temperature, mass flux and heat flux. The vertical smooth and micro-fin tubes with outer diameter (OD) of 5 mm and length of 1.44 m were selected as a test section to measure the evaporative heat transfer coefficient. The tests were conducted at mass fluxes from 212 to 530 kg/(m 2 s), saturation temperatures from −5 to 20 °C and heat fluxes from 15 to 45 kW/m 2, where the test section was heated by a direct heating method. The differences of heat transfer characteristics between the smooth and the micro-fin tubes were analyzed with respect to enhancement factor (EF) and penalty factor (PF). Average evaporation heat transfer coefficients for the micro-fin tube were approximately 111–207% higher than those for the smooth tube at the same test conditions, and PF was increased from 106 to 123%.

Experimental studies on the evaporative heat transfer and pressure drop of CO 2 in smooth and micro-fin tubes of the diameters of 5 and 9.52 mm

Carbon dioxide among natural refrigerants has gained a considerable attention as an alternative refrigerant due to its excellent thermophysical properties. In-tube evaporation heat transfer characteristics of carbon dioxide were experimentally investigated and analyzed as a function of evaporating temperature, mass flux, heat flux and tube geometry. Heat transfer coefficient data during evaporation process of carbon dioxide were measured for 5 m long smooth and micro-fin tubes with outer diameters of 5 and 9.52 mm. The tests were conducted at mass fluxes of from 212 to 656 kg m −2 s −1, saturation temperatures of from 0 to 20 °C and heat fluxes of from 6 to 20 kW m −2. The difference of heat transfer characteristics between smooth and micro-fin tubes and the effect of mass flux, heat flux, and evaporation temperature on enhancement factor (EF) and penalty factor (PF) were presented. Average evaporation heat transfer coefficients for a micro-fin tube were approximately 150–200% for 9.52 mm OD tube and 170–210% for 5 mm OD tube higher than those for the smooth tube at the same test conditions. The effect of pressure drop expressed by measured penalty factor of 1.2–1.35 was smaller than that of heat transfer enhancement.

A Study on Nucleate Boiling Heat Transfer Enhancement by Thermal Spray Coating

In order to enhance nucleate boiling heat transfer it is efficient to increase nucleation site density. This study deals with heat transfer enhancement surface manufactured by thermal spraying. Two thermal spraying methods using copper as a coating material, wire flame spraying (WFS) and vacuum plasma spraying (VPS), were applied to the outside of copper cylinder with 20 mm OD. The surface structure by WFS was denser than that by VPS. The heat transfer performance around the horizontal tubes under microgravity was evaluated in pool boiling experiments using HCFC123 for heat fluxes between 1.0 and 160 kW/m2 and saturation temperature of 30 °C. The microgravity experiments were carried out during a parabolic flight of an airplane. As a result, the surface by VPS produced higher heat transfer coefficient and lower superheat at boiling incipiency under microgravity. For the smooth surface, the effect of gravity on boiling heat transfer coefficient was small. For the coated surface, a large difference in heat transfer characteristics by gravity was observed in the moderate heat flux range. The heat transfer was deteriorated by the change from the normal gravity to hyper-gravity, while it was improved by the transition from the hyper-gravity to microgravity. The difference in heat transfer coefficient was a little between the normal gravity and microgravity. The effect of the coatings on the CHF was not observed.

A computer simulation of oxygen partial pressure and temperature profiles during hyperthermia.

Hyperthermia is a cancer therapy utilizing the difference in heat transfer characteristics between the tumor and normal tissue. In many cases, preferential heating of the tumor takes place because the perfusion rate is less in the tumor tissue relative to the normal tissue. This therapy has the advantage of fewer side effects during and after the treatment compared with other currently used cancer therapies. Hyperthermia can be used not only by itself and also in conjunction with other cancer therapies, such as chemo-therapy (Haranaka et al., 1987), x-irradiation (George et al., 1989; Fujiwara, et al., 1990), or both (Howard and Bleehen, 1988), to enhance tumor eradication or regression.

Experimental Study of Differences in Single-Phase Forced-Convection Heat Transfer Characteristics between Upflow and Downflow for Narrow Rectangular Channel

The differences in the single-phase forced-convection heat transfer characteristics between upflow and downflow were investigated experimentally with a narrow vertical rectangular channel. The objectives of the experiment were to investigate in both laminar and turbulent flow regions the applicability of existing correlations to and the effects of buoyant force on the heat transfer characteristics in the narrow vertical rectangular channel, which is simulating a subchannel of 2.25 mm in gap and 750 mm in length in the fuel element of the research reactor, JRR-3 to be upgraded at 20 MWt. As the results, it was revealed that (1) by use of equivalent hydraulic diameter, existing correlations are applicable to a channel as narrow as 2.25 mm in gap for turbulent flow though the precision and critical Reynolds number are different among the correlations, and (2) in the laminar flow, the difference in heat transfer characteristics arises between upflow and downflow with Reynolds number less than about 700 and Grashof number larger than about 1,000, giving smaller Nusselt number for down-flow than for upflow as the effect of buoyant force. New heat transfer correlations for channel heated from both sides are proposed as lower limits for upflow and downflow, respectively, in the laminar flow.

Experimental study of differences in single-phase forced-convection heat transfer characteristics between upflow and downflow for narrow rectangular channel.

The differences in the single-phase forced-convection heat transfer characteristics between upflow and downflow were investigated experimentally with a narrow vertical rectangular channel. The objectives of the experiment were to investigate in both laminar and turbulent flow regions the applicability of existing correlations to and the effects of buoyant force on the heat transfer characteristics in the narrow vertical rectangular channel, which is simulating a subchannel of 2.25 mm in gap and 750 mm in length in the fuel element of the research reactor, JRR-3 to be upgraded at 20 MWt. As the results, it was revealed that (1) by use of equivalent hydraulic diameter, existing correlations are applicable to a channel as narrow as 2.25 mm in gap for turbulent flow though the precision and critical Reynolds number are different among the correlations, and (2) in the laminar flow, the difference in heat transfer characteristics arises between upflow and downflow with Reynolds number less than about 700 and Grashof number larger than about 1,000, giving smaller Nusselt number for down-flow than for upflow as the effect of buoyant force. New heat transfer correlations for channel heated from both sides are proposed as lower limits for upflow and downflow, respectively, in the laminar flow.