Difference In Heat Transfer Coefficient Research Articles

Application of TiH2 dehydrogenation for vapour layer formation under boiling crisis conditions

This study explores TiH2 dehydrogenation for vapor layer formation under boiling crisis [1] conditions, aiming to reduce friction in the maritime sector. Despite the rise of electric vehicles for urban transport, marine vessels lack alternatives to fossil fuels. The research aims to combine compliant surfaces with two-phase flow generation, focusing on testing TiH2 (TH-2). Moreover, the study highlights the need for hydrogen-generating materials and suggests that material choices influence two-phase flow initiation conditions. After conducting experiments, it was determined that TiH2 affects the cooling dynamics: at the initial time moment, the cooling of the samples coated in TiH2 was more intense compared to polished aluminum samples (AL-1) due to the hydrophilic properties of the surface. When the TiH2-coated sample temperature dropped to approx. 200 °C, the cooling process would slow down, or the process would temporarily become isothermic (in some cases, a slight increase in temperature was observed as well) due to the formation of TiO2 or Al2O3, both of which are exothermic reactions. Dehydrogenation likely only takes place at the initial time moment and when the conditions for the Leidenfrost effect are favorable, which would serve to sustain the high temperature of the sample. It was concluded that the amount of accumulated hydrogen gas in cases of TH-2 samples was insufficient to cause a significant effect on critical Leidenfrost temperature. Nevertheless, a significant difference in heat transfer coefficient between AL-1 samples and TH-2 was observed due to physical–chemical processes on the TiH2 film.

Experimental investigation on boiling heat transfer characteristics of R1234yf/R1336mzz(Z) in horizontal flow

Due to the inclusion of 18 HFCs, including R134a and R245fa, in the Kigali Amendment as controlled working fluids, the search for excellent alternatives to R134a and R245fa is urgent. R1234yf and R1336mzz(Z) are potential alternative working fluids to R134a and R245fa, but most studies have focused on the comparison of thermodynamic performance and boiling heat transfer capability of these pure working fluids. There is limited research on the boiling heat transfer performance of their mixed working fluids. Therefore, this study focuses on the flow boiling heat transfer characteristics of the mixed working fluid R134a/R245fa under non-azeotropic conditions. Experimental methods were employed to investigate the boiling heat transfer performance of the alternative working fluids R1234yf/R1336mzz(Z) (including R245fa/R1234yf and R1234yf/R1336mzz(Z) mixed working fluids) and evaluate the feasibility of R1234yf and R1336mzz(Z) as substitutes for R134a and R245fa. The results indicate that among the three mixed refrigerants, R134a/R245fa exhibits the highest heat transfer coefficient, while R1234yf/R1336mzz(Z) shows the lowest heat transfer coefficient. However, the differences in heat transfer coefficients among these three are relatively small, and their overall trends under different operating conditions are similar. It is evident that R134a/R245fa, with its good thermodynamic performance and stability, still holds potential as a transitional working fluid. R1234yf/R1336mzz(Z), with similar physical and thermodynamic properties, is an excellent alternative to R134a/R245fa.

Experimental study on falling film evaporation heat transfer outside a horizontal tube in refrigeration system

Experimental study on falling film evaporation heat transfer outside a horizontal tube in refrigeration system

Electrowetting assisted shedding of droplets by a shearing airflow in dropwise condensation

Condensation is a necessary part of most industrial processes dealing with phase change. Efficient condensate removal (shedding) increases the systems’ performance. This study investigated the influence of electrowetting on droplet shedding and heat transfer enhancement during humid air condensation on a horizontal substrate. A mini closed-looped wind tunnel was used to simulate the condensation environment and control condensation parameters. AC electric field with the frequencies of 1 and 10 kHz was applied and compared with the control case (no voltage). The transient inverse heat conduction method was used to determine the heat transfer coefficient, and heat transfer measurements were synchronized to a high-resolution camera to monitor the condensation dynamics. The results show that the area-weighted average droplet radius was increased by 61 % and 84 % for 1 kHz and 10 kHz, respectively, compared to the control case for 1 m/s airflow. At 10 kHz, the shedding time was reduced, particularly at higher airflow velocities. Additionally, the heat transfer coefficient (HTC) exhibited a significant increase at 10 kHz when shedding was present. Notably, the difference in HTC became pronounced at an airflow velocity of 15 m/s, where shedding was prominent. In this scenario, the HTC rose by an impressive 35 % compared to the control case. These findings provide a promising foundation for utilizing EW to enhance heat transfer and improve condensate shedding under shear flow conditions.

Effect of Integrating Metal Wire Mesh with Spray Injection for Liquid Piston Gas Compression

Heat transfer enhancement techniques used in liquid piston gas compression can contribute to improving the efficiency of compressed air energy storage systems by achieving a near-isothermal compression process. This work examines the effectiveness of a simultaneous use of two proven heat transfer enhancement techniques, metal wire mesh inserts and spray injection methods, in liquid piston gas compression. By varying the dimension of the inserts and the pressure of the spray, a comparative study was performed to explore the plausibility of additional improvement. The addition of an insert can help abating the temperature rise when the insert does not take much space or when the spray flowrate is low. At higher pressure, however, the addition of spacious inserts can lead to less efficient temperature abatement. This is because inserts can distract the free-fall of droplets and hinder their speed. In order to analytically account for the compromised cooling effects of droplets, Reynolds number, Nusselt number, and heat transfer coefficients of droplets are estimated under the test conditions. Reynolds number of a free-falling droplet can be more than 1000 times that of a stationary droplet, which results in 3.95 to 4.22 times differences in heat transfer coefficients.

The Investigation on Heat Transfer Characteristics of Steam Condensation in Presence of Noncondensable Gas under Natural Convection

The NHR-200 reactor in China adopts the noncondensable gas self-stabilizing control and the noncondensable gas used for pressure stabilization control can weaken steam condensation heat transfer in the integrated steam-gas pressurizer. A condensation experimental system was established and the heat transfer characteristics of steam-nitrogen and steam-argon condensation under natural convection had been investigated. The pressure ranged from 0.516 to 5.10 MPa. The distributions of nitrogen and argon in the steam/gas mixture were obtained in the experiments, and the results showed that nitrogen and argon were evenly distributed in the steam under different pressure, respectively. The effects of heat transfer temperature difference had also been investigated and it is found that the total heat transfer coefficient difference had little influence on the total condensation heat transfer coefficient. However, the steam condensation heat transfer coefficient decreased with the increase of the degree of supercooling of the wall. The condensation heat transfer coefficient was reduced by approximately 0.11 kW/(m2·K) as the degree of supercooling of the wall changed from 14°C to 36°C. The condensation heat transfer coefficient also decreased with the mass/molar fraction of noncondensable gas increasing and a certain difference between the effect of the mass fraction of noncondensable gas and the effect of the molar fraction of noncondensable gas was discussed in this paper.

Analysis of heat transfer coefficients for simulation of the heat exchange between oil and the internal cavity faces of the isolated piston at variable gravity conditions

The paper deals with the problems related to the development of a new piston design for replacement during repair in 2-stroke D-100 engines for locomotives. The paper addresses the design, the mass of which is 35 % less than the mass of the prototype. The results of thermal calculation, which served the initial data for the temperature and heat transfer coefficients evaluation over the heat-receiving surface of the combustion chamber of the piston, coincide with the experiment of prof. G. Rosenblit. The integral boundary conditions were refined by calculating five successive engine cycles, αP = 1830 W m–2 K–1 and Teq p = 970 K were selected for further research. Next, heat transfer from the piston internal cavity to the oil, which is supplied through a nozzle in the small head of the connecting rod, was studied. The study included a 3D simulation of the cooling oil movement from the nozzle to the inner surface, both in the stationary mode (Earth’s gravity field) and taking into account the variable acceleration of the piston at the design mode. In this case, the location of the holes in the nozzle, the flow parameters and the properties of the oil were simulated. The study showed that taking piston acceleration into account leads to a slight difference in heat transfer coefficient in the center of the piston (830–800 W m–2 K–1) and rather large at the periphery (600 … 520 W m–2 K–1 taking into account the acceleration of the piston against 460 … 420 W m–2 K–1). It is shown that the nozzle with 4 holes and the angle of inclination of the side holes φ = 55° gives a more efficient cooling.

Performance analysis on the charging and discharging process of a household heat pump water heater

In this study, a household heat pump water heater (HPWH) test system was built and a coupling model containing both vapor-compression cycle and water heater was established to analysis the charging and discharging performance. Water temperature distribution, heat transfer coefficient and operating efficiency of the system were three main characteristics to compare the performance of HPWH with fixed/varied-diameter coil. The results of the coupling simulation showed good agreement with experimental results. In the water-charging process, heat transfer coefficient and COP of varied-diameter coil were 20.00% and 10.23% higher than that of fixed-diameter coil, respectively. In water-discharging process, difference of heat transfer coefficient and COP between these two coil designs was relatively small. However, the discharging efficiency and extraction efficiency of varied-diameter coil were 7.74% and 9.70% higher than that of fixed-diameter, respectively. Thus, this work could provide some guideline on optimization of condenser coil designs.

New analytical solutions of wellbore fluid temperature profiles during drilling, circulating, and cementing operations

Historically, the formulation of the wellbore temperature profiles during drilling, circulating, and cementing operations relied upon the boundary condition or BC at the bottomhole location, wherein the tubular and annular temperatures become equal. This study presents two other formulations to explore alternative to bottomhole BC's that may potentially occur in wellbores.Application of the energy balance in the wellbore for both forward and reverse circulations underpins all three formulations. The wellbore temperature profiles generated by implementing the energy balance in the system depends on the type of the boundary condition used. Initially, we generated various temperature profiles using these models. In general, we observed that the maximum temperature occurs at the end of the tubular or some distance away from the bottom. For open and cased-hole sections, the difference in heat-transfer coefficients triggers different magnitudes of heat transfer and affects the corresponding temperature profiles.We compared the performance of our models with those presented in the literature. Thereafter, we sought to validate the models with diverse set of field data. Given the ability of all three models to handle changes in the geothermal gradient due to the characteristics of the sediments, salt domes, and gas hydrates, we explored their performance in situations where there is significant heat transfer. The holistic approach pursued here provided the necessary insights into various temperature profiles in a given situation.

Numerical study of buoyancy’s effect on flow and heat transfer of kerosene in a tiny horizontal square tube at supercritical pressure

The flow and heat transfer characteristics of China No. 3 aviation kerosene (RP-3) flowing in a tiny horizontal square tube at supercritical pressure under gravity are numerically studied, focusing mainly on the secondary flow and buoyancy effects on heat transfer at different flow parameters. The heated length of the tube is 480 mm while the inner length of side is 2 mm. Results indicate that buoyancy will cause secondary flow, and the intensity of secondary flow decreases with mass flow rate and pressure, while increases with volume heat source density. The heat transfer coefficient difference between the walls also decreases with mass flow rate and pressure, while increases with volume heat source density. Temperature of the top wall is higher than that of the bottom wall, while the average heat flux of the top wall is lower than that of the bottom wall. Heat transfer coefficient of the bottom wall is the highest, while that of the top wall is the lowest. Jackson’s criterion and Petukhov’s criterion were tested to assess the application of the two criteria in the present study. Results indicate that Jackson’s criterion can be used to judge the buoyancy effect quantitatively while Petukhov’s criterion can not.

Condensation heat transfer and pressure drop characteristics of R-134a in horizontal smooth tubes and enhanced tubes fabricated by selective laser melting

This paper reports a study of condensation heat transfer and pressure drop of R134a inside four enhanced tubes and one plain tube fabricated by Selective Laser Melting (SLM). The results are compared to a plain commercial aluminum tube. The enhanced tubes consist of a tube with a metallic foam structure, a tube with eight short circumferential pin fins, a tube with five long circumferential pin fins and a tube with five twisted pin fins. The experiments were conducted at mass fluxes from 50 to 150 kg/m2 s. Throughout the experiments, the inlet and outlet vapor qualities were maintained at 0.9 and 0.3, respectively. Two saturation pressures of 13.4 bar and 11.6 bar were investigated. The effects of fin height, refrigerant flow direction and mass flux on the heat transfer coefficient and pressure drop were studied. Our results show that for the wavy flow pattern, the saturation pressure, vapor quality, mass flux, refrigerant flow direction and fin structure have significant effects on the condensation heat transfer coefficient and pressure drop. At higher saturation pressures, the head impact on the fins with shorter fin height has a higher heat transfer coefficient than the back impact. For the longer and twisted fins, a reversed trend was observed. With an increase in the mass flux or a decrease in the saturation pressure, the difference in heat transfer coefficients between the head and back impact for the same tube structure reduces. The heat transfer coefficients of the metallic foam tubes are higher than that of the plain SLM tube with a large penalty of pressure drop. The eight-fin tubes yield higher efficiency indices in terms of heat transfer over pressure drop when compared to the other tubes.

Assessment of body force effects in flow condensation, Part I: Experimental investigation of liquid film behavior for different orientations

Body force effects in flow condensation vary depending on channel orientation and fluid mass velocity, making the design of systems intended to operate in multiple orientations more complicated than those at a fixed orientation. This study examines the effects of body force on liquid film development for flow condensation of FC-72 in horizontal, vertical upflow, and vertical downflow orientations. Two test sections are utilized, one capable of providing high-speed imaging of liquid film development, and the other designed to allow detailed measurements of flow condensation heat transfer coefficient. High speed imaging shows that for low FC-72 mass velocities, flow regimes differ significantly among the three orientations, with vertical upflow exhibiting falling film behavior, horizontal flow showing stratification, and vertical downflow displaying annular co-current flow. For the case of low mass velocity horizontal flow, interfacial disturbances in the form of a sinusoidal wave are clearly visible with wavelengths on the order of 1–10mm. As mass velocity is increased, the liquid film is seen to exhibit similar behavior for all three orientations due to interfacial shear stress negating body force effects. Heat transfer measurements reinforce these trends, with circumferential variations in heat transfer coefficient present for horizontal flow at low mass velocities, and differences in the axial variations in heat transfer coefficient seen when comparing vertical upflow to vertical downflow. As mass velocity is increased, differences in heat transfer coefficient are reduced, with the highest mass velocities exhibiting almost no variation with orientation. This convergence of values indicates the ability of interfacial shear stress to mitigate body force effects at sufficiently high mass velocities.

산질화 표면에서 액적의 증발열전달 특성

Abstract The present study aims to experimentally investigate the evaporative heat transfer characteristics of Oxi-nitriding SPCC sur-face. Moreover, the heat transfer coefficient was examined with respect to surface temperature during droplet evaporation. Infact, the nitriding surface showed significant enhancement for anticorrosion performance compared to bare SPCC surface butthe thermal resistance also increased due to the formation of compound layer. From the experimental results, the evaporativebehavior of sessile droplet on nitriding surface showed similar tendency with the bare surface. Total evaporation time ofsessile droplet on the nitriding surface was delayed less than 5%. The difference in heat transfer coefficient increased withthe surface temperature, and the maximum difference was estimated to be around 11% at 80 o C surface. Thus, this nitridingsurface treatment method could be useful for seawater heat exchanger industries. 기호설명 ρ 액체의 밀도, kg/m 3 θ 동적 접촉각, degθ ECA 평형 접촉각, degA 액적의 접촉면적, m

Numerical investigation on blade leading edge high-efficiency swirl and impingement phase transfer cooling mechanism

ABSTRACTA comparative study of the flow field and heat transfer characteristics between swirl and impingement of mist/air cooling on blade leading edge is carried out to find a better cooling configuration for phase transition cooling. The Eulerian–Lagrangian particle tracking technique is used to investigate mist/air cooling. Comparisons are made between these two cooling forms in such aspects as vortex structure, heat transfer enhancement, pressure loss, and thermal uniformity with and without mist injection. The influences of mist ratio and Reynolds numbers on these parameters are studied in this paper. Results show that heat transfer is enhanced while pressure loss and thermal uniformity are improved by the swirl flow created by vortex impingement. The heat transfer performance increases by about 46.2 and 51.9%, respectively, for impingement and swirl cooling with 8% mist injection, and the pressure loss coefficient increases by 19%. The difference of heat transfer coefficient between swirl and impingement cooling with and without mist injection at high Reynolds number is larger than that at low Reynolds number. In addition, the heat transfer nonuniform coefficient of swirl cooling is about 15% lower than for impingement cooling.

Conjugate Jet Impingement Heat Transfer Investigation via Transient Thermography Method

Investigating the conduction–convection coupling, the present study is focused upon measurement of conjugate heat transfer ensuing jet impingement on a 15-mm-thick metallic plate. Based on a rapid change in jet temperature and using time-accurate infrared thermography, a transient measurement methodology is developed toward acquisition of heat transfer coefficients. The new technique is shown to have comparable levels of Nusselt number and effectiveness accuracy, all while significantly reducing the number of consecutive measurements and their duration. To highlight the significance of the conjugate effect, different plate materials (copper, steel, and Inconel) are employed to differ the solid thermal conductivities, resulting in Biot-number variations. The plate surface heat transfer is studied at two injection Reynolds numbers (34,000 and 37,000) and for two nozzle-to-plate distances (two and five jet diameters). The changes in slab material conductivity reveal small but quantifiable differences in heat transfer coefficients (up to locally and 9% globally). Furthermore, constituting an upper bound and lower bound, respectively, it is observed that all conjugate Nusselt number distributions lay within the two limits of isoheat flux and isothermal boundary condition cases.

Investigation of the two-phase convective boiling of HFO-1234yf in a 3.9 mm diameter tube

In this study, the influences of heat flux and mass flux on the two-phase convective boiling heat transfer performance are reported for refrigerants HFO-1234yf and HFC-134a in a 3.9mm smooth diameter tube. Tests are performed with a saturation temperature of 10°C. It is found that at lower vapor quality region the nucleate boiling is the dominant heat transfer mechanism while the convective evaporation mechanism takes control at the higher vapor quality region. Both HFC-134a and HFO-1234yf shows similar trend and the difference in heat transfer coefficient between HFO-1234 and HFC-134a is quite small. The comparable heat transfer performance between HFC-134a and HFO-1234yf is attributed to similar physical properties and nucleate boiling contribution. The present test results are in line with some existing reports but are inconsistent with one other study having a tube diameter of 1.1mm. It is found that the departure of heat transfer coefficients between the available publications is mainly attributed to the different flow phenomena caused by the difference of the channel size and channel geometry. A noticeable deterioration of the heat transfer coefficient for HFO-1234yf is encountered in the microchannel. The pressure drops for HFC-134a is about 5–15% higher than that of HFO-1234yf.

Simulation of Solidification for Metal Matrix Composites

The paper use the finite element method, simulating the solidification process of metal matrix composite. Obtain the changing of temperature field about the solidification process and some temperature curve for special nodes. The results show that, due to the difference of heat transfer coefficient about the matrix and the metal the solidification process for composite materials showed the irregular temperature cloud, namely nonlinear temperature distribution. The simulation actually provides some guidance for synthesis of metal composite materials.

Experimental study of falling film evaporation heat transfer outside horizontal tubes

The heat transfer process of falling film horizontal evaporation includes evaporation outside tubes and condensation inside tubes, the heat transfer coefficient of the former is about 50% of that of the latter. So the overall heat transfer coefficient is influenced mainly by the falling film evaporation outside tubes. An experimental study of falling film heat transfer outside horizontal tubes was carried out in order to show how the heat transfer coefficient is affected by different parameters such as flow density evaporation temperatures, temperature difference between wall and saturation water, and mass concentration of the seawater. Experiments were conducted using 14 mm outer diameter Al-brass tubes heated by internal electric heaters so that a uniform heat flux was generated on the outside surface of tubes. The results show that when flow density Γ varies between 0.013 kg/ms <Γ<0.062 kg/ms, the heat stransfer coefficient of falling film evaporation outside horizontal tubes h increases with the increase in liquid feeding, evaporation boiling temperature and heat flux. h also increases with an increase in distributor height, however there is a maximum height in which any height above this. Besides, the amount of non-condensing gas has significant effect on h. The difference of heat transfer coefficient between freshwater and seawater is small. These results contribute to further improving the performance of heat transfer process and developing new evaporator.

Effect of gravity on pool boiling heat transfer on thermal spray coating

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 effect of gravity on boiling heat transfer coeffcient and wall superheat at the onset of boiling were experimentally evaluated under micro- and hyper-gravity condition during a parabolic trajectory flight of an airplane. Pool boiling experiments in saturated liquid of HCFC123 were carried out for heat fluxes between 1.0 and 160 kW/m2 and saturated temperature of 30 °C. As a result, the surface by VPS produced higher heat transfer coefficient and lower superheat at the onset of boiling under microgravity. For the smooth surface, the effect of gravity on boiling heat transfer coefficient was a little. For the coating, a large difference in heat transfer coefficient to gravity was observed in the moderate heat flux range. The heat transfer coefficinet decreased as gravity changed from the normal to hypergravity, and was improved as gravity changed from the hyperto microgravity. The difference in heat transfer coefficient between the normal and microgravity was a little. Heat transfer enhancement factor was kept over the experimental range of heat flux. It can be said that boiling behavior on thermal spray coating might be influenced by flow convection velocity.

고정된 터빈 블레이드의 베인에 대한 상대위치 변화가 끝단면 및 슈라우드의 열/물질전달 특성에 미치는 영향

The effect of relative position of the stationary turbine blade for the fixed vane has been investigated on blade tip and shroud heat transfer. The local mass transfer coefficients were measured on the tip and shroud for the blade fixed at six different positions within a pitch. A low speed stationary annular cascade with a single turbine stage was used. The chord length of the tested blade is 150 ㎜ and the mean tip clearance of the blade having flat tip is 2.5% of the blade chord. A naphthalene sublimation technique was used for the detailed mass transfer measurements on the tip and the shroud. The inlet flow Reynolds number based on chord length and incoming flow velocity is fixed to 1.5×10<SUP>5</SUP>. The results show that the incoming flow condition and heat transfer characteristics significantly change when the relative position of the blade changes. On the tip, the size of high heat/mass transfer region along the pressure side varies in the axial direction and the difference of heat transfer coefficient is up to 40% in the upstream region of the tip because the position of flow reattachment changes. On shroud, the effect of tip leakage vortex on the shroud as well as tip gap entering flow changes as the blade position changes. Thus, significantly different heat transfer patterns are observed with various blade positions and the periodic variation of heat transfer is expected with the blade rotation.