Heat Transfer Coefficient In Region Research Articles

The bubble merger in rectangular microchannels during boiling processes based on conjugate heat transfer

In recent years, boiling heat transfer in microchannels has become an effective solution for managing high heat flux in electronic devices. Numerical simulations offer details on flow characteristics that experimental methods cannot capture, deepening our understanding of boiling mechanisms within microchannels. In this study, we developed a self-programmed solver, “interMultiRegionFoam,” based on the volume of fluid (VoF) method, phase-change mass transfer models, and conjugate heat transfer. Analyzing the force acting on individual bubbles, a model for two bubbles' merger was constructed, encompassing three stages: bubble bridging, bubble retraction, and bubble contraction. For isolated bubbles, during the restricted growth stage, the dry patch length increases by only 25% with an increase in the Capillary number, while in the elongation growth stage, it grows fivefold. Furthermore, the investigation into four typical bubble merger cases within microchannels demonstrated that the bubble merger model effectively describes the process. During the restricted and free growth stages, bubble merger was observed to enhance heat transfer, and the heat transfer coefficient in the bottom region of the macrolayer increased by a maximum of 16%. However, when bubbles merge in the elongation growth stage, the maximum increase in the heat transfer coefficient at the bottom of the macrolayer is only 7%. Several factors contribute to the enhanced heat transfer during bubble merger, including the evaporation and rewetting of macrolayer liquid, as well as the replenishment of the microlayer. Moreover, the disturbance caused by bubble merger and transient conduction arising from bubble retraction and contraction contributes to the enhancement in heat transfer. With an increase in the contact angle, the peak heat flux gradually decreases, with a maximum reduction of 27%. As the heat flux increases, the Nusselt number gradually increases, reaching a maximum peak increase of 52%. These studies provide theoretical guidance for the design of wall modifications and nucleation in microchannels.

Analysis of Confined Jet Impingement in Converging Annular Microchannel Heat Sinks

Jet impingement cooling is deemed an excellent choice for the thermal management of high-power electronics. However, high-pressure drop penalties and low local heat transfer coefficients in regions far from the jet zone are its drawbacks. Although it is reported that recirculation areas appear because of the entrainment, the effects of recirculation size on thermal behavior are not understood well enough. Here, jet impingement heat sinks with converging annular channels are employed in a numerical investigation to minimize the adverse cooling effects associated with an impinging jet in a microchannel. The realizable k−ε turbulent model is used for modeling thermal and turbulent flow fields (Re=5,000 to 25,000). It was found that the different flow recirculation zones in small scales are responsible for the enhanced heat transfer rate. While the thermal performance of a converging wall jet impingement heat sink is higher than its flat wall counterpart at low Re numbers, the thermal performance results are in favor of the flat wall jet impingement heat sink at high Re numbers. The flow recirculation area shrinks in converging channels at high Re numbers, thereby deteriorating the thermal performance of the converging channel compared with a flat wall jet heat sink. Also, it was found that employing steeper converging channels shrinks the flow recirculation region, resulting in up to 59% lower pressure drops at Re=25,000. The present study examines the role of flow recirculation at different Re numbers on the thermohydraulic performance of jet impingement converging annular heat sinks.

Methodology for determining convective heat transfer coefficients on the tubesheet surface of shell-and-tube heat exchangers under single-phase flow

Designing shell-and-tube heat exchangers requires reliable methods to calculate the convective heat transfer coefficient (CHTC) on the tubesheet, due to its impact on thermal stress. To address this, we established numerical models for both the tube and shell sides of the tubesheet. Using computational fluid dynamics (CFD), we systematically investigated the distribution of convective heat transfer on the tubesheet surface and its variations with structural and process parameters. The results showed that when fluid flowed into the tubesheet, the average convective heat transfer coefficient on the tube side of the tubesheet surface was 26% higher than when it flowed out. Additionally, the average convective heat transfer coefficient in the perforated region of the tubesheet was 1.39 to 2.57 times greater than in its periphery. Furthermore, the local CHTC on the shell side of the tubesheet surface gradually decreased along the downstream direction from the inlet of the shell-side fluid. Through simulations conducted across various structural and process parameters, we derived empirical formulas to rapidly calculate the average CHTC on the tubesheet surface. These formulas incorporated a tubesheet porosity factor for the tube side and introduced a new method for calculating the equivalent tubesheet diameter on the shell side. Random testing verified that our proposed formula reliably predicted the numerical simulation results.

Construction of Building an Energy Saving Optimization Model Based on Genetic Algorithm

The building envelope structure is the cause of a major part of energy consumption, with exterior walls and windows being the main energy-consuming components. Traditional building energy conservation measures often overlook the demand for human comfort, especially in areas characterized by hot summers and warm winters. For this paper, the authors concentrated on indoor comfort, with a focus on optimizing the heat transfer coefficient of windows and exterior walls using a genetic algorithm. They used a genetic algorithm to explore the performance optimization of exterior walls and windows in the enclosure structure. To aid this effort, they constructed a building energy-saving optimization model. In addition, they created an optimization model in an attempt to reduce building energy consumption. They took the heat transfer coefficients of the outer window and the outer wall as the optimization parameters of the established model, and they compiled the optimization program using MATLAB. The experimental results showed that the heat transfer coefficients of exterior walls and windows in cold regions are 0.4459 and 2.7875, respectively, while the heat transfer coefficients in warm winter and hot summer regions are 0.66, 1.98, 1.026, and 1.59. The conducted work provides a reference for the optimization design of the heat transfer coefficient of external walls and windows as a measure to enhance building energy efficiency.

Design and Off-design Performance Analysis of a Zigzag Channeled Precooler for Indirect Cooling System of Supercritical CO2 Recompression Cycle Incorporated with a Flow-Bypass System

The employment of a Printed Circuit Heat Exchanger (PCHE) as a Precooler in an indirect cooling system of a supercritical CO2 (sCO2) power block is the prevailing trend in research since cooling the working fluid with water offers superior cooling performance compared to an air-cooled system. The enhanced cooling performanceofthe Printed Circuit Heat Exchanger offers the desired sCO2 compressor inlet temperature close to the pseudo-critical point, maximizing the thermodynamic efficiency of the cycle. The improbable fluctuating properties of sCO2 near the critical region make it arduous to design a component that operates near the critical region. However, the high heat transfer coefficient in the near-critical region increases the power cycle’s performance and reduces the cycle components’ size, making it cost-effective. In the indirect cooling system of the sCO2 recompression cycle, the working fluid (sCO2) is cooled by cooling water in a Printed Circuit Heat Exchanger known as the Precooler. Thus, the heat gained by the cooling water is cooled by the air-cooled heat exchanger unit inside a Natural draft dry cooling system. In this study, an iterative nodal approach is implemented to develop a Python code for designing a Zigzag channeled Printed Circuit Heat Exchanger as the Precooler for the indirect cooling system of the sCO2 recompression cycle for the Concentrated Solar Power application. The off-design performance study is conducted under different sCO2 inlet temperatures (338 K–378 K), inlet pressure (7 MPa–10 MPa), and a wide range of cooling water inlet temperatures (283 K–318 K), suggesting the heat transfer rate can be enhanced by lowering the water inlet temperature and sCO2 working pressure. Nevertheless, the variation in sCO2 input temperature has little influence on the precooler heat transfer rate. The result also demonstrates that the pressure drop on the sCO2 side decreases upon increasing these parameters. The off-design performance improvement during low ambient temperature is achieved by introducing a flow bypass in the power block, which optimizes the precooler sCO2 mass flow to maintain the desired compressor inlet temperature.

Heat Transfer Intensification Mechanism of a Baffled Fluidized Bed Heat Exchanger

Gas–solid fluidized bed heat exchangers are commonly applied to remove heat in highly exothermic processes. The heat transfer performance is related to hydrodynamics. Baffles, such as ring-type baffles, louver baffles, and packings, are widely applied to improve the hydrodynamics in a fluidized bed. In this study, a baffled fluidized bed heat exchanger was proposed to enhance the bed-to-surface heat transfer coefficient. The baffles are specially designed to intensify the contact characteristics between heat transfer tubes and fluidized bed medium. The hydrodynamics and heat transfer characteristics were compared between the conventional and the newly designed baffled heat exchanger to explore the intensification mechanism in dilute and dense fluidization regions. Proper arrangement of the baffles in the fluidized bed heat exchanger increases the time fraction of the particle packet as well as the solid holdup, which enhances the heat transfer coefficient in dilute region. For the dense region, the contact time between the packet and heat transfer surface is decreased in the baffled fluidized bed heat exchanger due to the reduced bubble size, increased bubble frequency, and enhanced radial movement of bubbles and particles. Therefore, the bed-to-surface heat transfer in the dense region is significantly intensified.

Ultrahigh evaporative heat transfer measured locally in submicron water films

Thin film evaporation is a widely-used thermal management solution for micro/nano-devices with high energy densities. Local measurements of the evaporation rate at a liquid-vapor interface, however, are limited. We present a continuous profile of the evaporation heat transfer coefficient (h_{text {evap}}) in the submicron thin film region of a water meniscus obtained through local measurements interpreted by a machine learned surrogate of the physical system. Frequency domain thermoreflectance (FDTR), a non-contact laser-based method with micrometer lateral resolution, is used to induce and measure the meniscus evaporation. A neural network is then trained using finite element simulations to extract the h_{text {evap}} profile from the FDTR data. For a substrate superheat of 20 K, the maximum h_{text {evap}} is 1.0_{-0.3}^{+0.5} MW/text {m}^2-K at a film thickness of 15_{-3}^{+29} nm. This ultrahigh h_{text {evap}} value is two orders of magnitude larger than the heat transfer coefficient for single-phase forced convection or evaporation from a bulk liquid. Under the assumption of constant wall temperature, our profiles of h_{text {evap}} and meniscus thickness suggest that 62% of the heat transfer comes from the region lying 0.1–1 μm from the meniscus edge, whereas just 29% comes from the next 100 μm.

Experimental Investigations of the LED Lamp with Heat Sink Inside the Synthetic Jet Actuator

The paper presents the experimental research on the thermal management of a 150 W LED lamp with heat sink inside a synthetic jet actuator. The luminous flux was generated by 320 SMD LEDs with a nominal luminous efficacy equal to 200 lm/W mounted on a single PCB. Characteristic temperatures were measured with three different measurement techniques: thermocouples, infrared camera, and an estimation of the junction temperature from its calibrated dependence on the LED forward voltage. The temperature budget between the LED junction and ambient as well as the thermal resistance network was determined and analyzed. The energy balance of the LED lamp is presented along with the values of the heat flow rate and heat transfer coefficient in different regions of the LED lamp surface. For an input power supplied to the SJA equal to 4.50 W, the synthetic jet dissipated approximately 89% of the total heat generated by the LED lamp. The heat from the PCB was transferred through the front and rear surfaces of the board. For the input power of 4.50 W, approximately 91% of the heat generated by LEDs was conducted by the PCB substrate to the heat spreading plate, while the remaining 9% was dissipated by the front surface of the PCB, mostly by radiation. The thermal balance revealed that for the luminous efficacy of the investigated LEDs, approximately 60% of the electrical energy supplied to the LED lamp was converted into heat, while the rest was converted into light.

Cooling characteristics of hot rolled seamless steel tube by jet impingement

The flow and temperature field variation of hot rolled seamless steel tube with outer diameter of 140 mm and wall thickness of 10 mm were studied by experiment and numerical simulation. The finite element model of circular jet impinging cooling steel tube was established, and the steel tube was divided into different regions in circumferential direction. It was found that the uniformly arranged multiple jets have non-uniform cooling characteristics under the influence of gravity and hollow characteristic. Under the condition of 12 jets, the average heat transfer coefficient in circumferential direction of −15 ∼ −45° region is 8385 W/(m2K), and the fluctuation of the average heat transfer coefficient in different regions reaches 3222 W/(m2K). According to statistical average cooling rate of the experiment, the average cooling rate increases by 34% when 2 jets are increased to 4 jets. When the number of jets exceeds 8, increasing the number of jets has a limited effect on improving average cooling rate. Through reasonable configuration of jet velocity of different nozzle positions, a way to realize uniform cooling was proposed and preliminarily verified. The improved average heat transfer coefficient fluctuation of circumferential was limited to 1000 W/(m2K), which significantly improved the uniformity of jet cooling.

Thermal performance of a central-jetting microchannel heat sink designed for a high-power laser crystal

Thermal management has become one of the critical problems in high-power laser systems. The laser crystal, which dissipates a heat flux over 150 W/cm2, is the most challenging component when solving the thermal problem. In this work, a specialized central-jetting microchannel heat sink (MCHS) is proposed purposely for a small-size laser crystal with high heat fluxes. Considering the space constraint and installation restriction, the MCHS has a narrow width to fit with the laser crystal, while its length is extended to enhance the heat dissipation capability. The MCHS is integrated by welding the two layers completely through a vacuum brazing method. Central-jetting impingement is introduced to enhance the cooling efficiency. Coolant is flushed vertically into the microchannel layer toward the central heating region where the heating source is attached at the central bottom of the MCHS. We conduct a parametric study on the thermal performance of the MCHS by experiments. The overall heat transfer coefficient of the MCHS achieves 50 × 103 W/(m2•K), and the average heat transfer coefficient of the microchannel reaches 21.5 × 103 W/(m2•K). The surface temperature of the heating head is controlled below 65 °C, and the temperature variation is less than 5 °C. In the solid bottom of the MCHS, thermal resistance in vertical direction is over 1.8 times of that in the horizontal direction, ensuring an efficient heat dissipation in the extended volume. The local heat transfer characteristics are further examined through additional numerical simulations. The wall temperature of the microchannel and the local heat transfer coefficient have a high plateau in the heating region, and then drop significantly along the flow direction. Due to heat transfer enhancement by jet impingement, the heat transfer coefficient in the central region is generally 15% higher than that averaged on the full microchannel.

Effects of electric field on flow boiling heat transfer characteristics in a microchannel heat sink with various orientations

This study investigates the effects of the electric field in an inclined microchannel heat sink with a hydraulic diameter of 2 mm. The saturated flow boiling performance of a dielectric liquid R141b is experimentally characterized. An electrode fabricated with enameled copper wires is placed above the medium and energized to generate an electric field between it and the heated surface with the applied voltage from 0 V to 200 V. The thermal performance, with/without electric field, under various orientations from 0° to 90°, is evaluated at an inlet mass flux of 51.58 kg/(m2·s) and initial temperature of 299.15 K. Results show that the onset of nucleation boiling is delayed by the electric field. Electric field brings more production and rotary/lateral motion of bubbles which contributes to advance the flow pattern transition. Under the electric field, elongated bubbles burst and some vortexes appear in the liquid film, which has been recorded by the high-speed video camera. Overall, the electric field can remarkably promote the flow boiling heat transfer coefficient (HTC) in lower vapor quality regions, and the maximum enhancement ratio is obtained as 1.48 under present conditions. However, the microchannel prematurely dries out due to the electric field. In this research, flow boiling HTCs are sensitive to orientation. With a constant applied voltage, the performance declines between 0°∼90° while a special transition happens between 45°∼60° Based on the Gungor-Winterton correlation, and revised with the exponential function as well as a non-dimensional number representing the electric field force, a new HTC asymptotic model is developed by first considering the effects of the electric field with an error of ±15% to the experimental data.

Heat Transfer Characteristics Analysis on a Fully Cooled Vane With Varied Density Ratios

Abstract The objective of this experimental investigation was to determine the cooling performance of a fully cooled vane with 18 rows of cylinder cooling holes. The exit Reynolds number in the wind tunnel normalized by the true chord was 500,000 with an inlet turbulence intensity of 15%. The film cooling effectiveness and heat transfer coefficient distribution were obtained by the transient liquid crystal (TLC) technology, three mass flow ratios (MFRs = 7.0%, 9.9%, 11%) and two density ratios (DRs = 1.0, 1.5) were tested. The results show that the film cooling effectiveness distribution on the suction side is more uniform and the coolant injection trajectory is much longer than that on the pressure side. As the density ratio increasing to 1.5, the more laterally uniform film cooling effectiveness contour on the pressure side is observed and the spatially averaged film cooling effectiveness is increased by 11–43%. For the MFR = 7.0%, the coolant injection with low momentum thickens the boundary layer and reduces the heat transfer coefficient in the mid-chord region of the pressure side. Both the increased mass flow ratio and decreased density ratio result in a higher heat transfer coefficient, while do not alter the distribution trend. By calculating the heat flux ratio, the reduction in the heat flux at DR = 1.5 is found to be within 20% in most areas than that of DR = 1.0 on the vane surface.

Effects of non-isothermal oxidation on transient conjugate heat transfer of the cryo-supersonic air-quenching

In this paper, the effects of non-isothermal oxidation on transient conjugate heat transfer of the cryo-supersonic air-quenching are investigated based on a double-layered oxidation kinetics model, while a unified conjugate heat transfer formula is developed to synthetically consider the near-wall turbulence, non-isothermal oxidation, and surface radiation. The comparison between numerical and experimental results are also presented to check the validity of the developed model. The results indicate that the film growth has some degree of inhibition to the conjugate heat transfer, in particular, the stagnation temperature increases linearly by about 5 K per 100 ?m increase in film thickness, and the effective conjugate heat transfer coefficient in the stagnation region decreases linearly by about 55 Wm?2K?1 per 100 ?m increase in film thickness. Moreover, the oxide film would have little impact on transient conjugate heat transfer when the near-wall velocity is higher due to the effect of viscous dissipation.

Enhanced flow boiling performance in high-aspect-ratio groove-wall microchannels

The dry-out phenomenon easily occurs on the side walls in high aspect ratio (channel depth to width ratio, AR) microchannel due to the decreasing of liquid film thickness. It is desirable to design better microchannels to address this issue. In this study, we proposed a new high-aspect-ratio groove-wall microchannel (Height=200 μm, Width=80 μm, Length=10 mm, AR=2.5), which features a series of rectangular grooves etched on the plain side walls. Flow boiling experiments were conducted in the novel groove-wall microchannel and conventional plain-wall microchannel at two inlet subcoolings of 40 and 70 °C, three mass fluxes (G) of 446, 630 and 815 kg/m2·s and wall heat fluxes of 3.8–139.5 W/cm2, using deionized water as the working fluid. The outlet of the channel is connected to the atmosphere, maintaining a saturation temperature of 100 °C. Flow visualization was captured by a high-speed camera to better understand the flow and heat transfer characteristics. Compared with plain-wall microchannel, the groove-wall microchannel demonstrates earlier boiling incipience, higher heat transfer coefficient and lower pressure drop in the two-phase flow region. It was found that the groove-wall microchannel obtains better heat transfer coefficient (HTC) at higher inlet subcooling. In comparison with plain-wall microchannel, the HTC enhancements of 25.4%, 73.0% and 55.0% were obtained respectively for the mass fluxes of 446, 630 and 815 kg/m2·s at ΔTsub=70 °C, while the corresponding HTC enhancements are 25.0%, 36.5% and 56.7% at ΔTsub=40 °C, respectively. Since the groove walls characterize the thin film evaporation especially at high heat flux, an enhanced critical heat flux (CHF) was obtained. At G = 446, 630 and 815 kg/m2·s, the groove-wall microchannel enhances the CHF respectively by 26.5%, 31.1% and 45.0% at ΔTsub= 40 °C, and the corresponding enhancements are 5.3%, 10.3% and 10.0% at ΔTsub=70 °C, respectively. Furthermore, flow reversal and boiling instability (pressure drop and wall temperature fluctuations) were drastically suppressed. Boiling instabilities are more severe at high inlet subcooling. The maximum pressure drop reductions of 27.4%, 42.0% were achieved in groove-wall microchannels for the mass fluxes of 630 and 815 kg/m2·s respectively at ΔTsub= 40 °C, and the corresponding maximum reductions are 9.07%, 23.6% at ΔTsub=70 °C, respectively. Almost no difference of the pressure drop was found in two microchannels at G = 446 kg/m2·s.

Thermal and electrical performances of semi-transparent photovoltaic glazing integrated with translucent vacuum insulation panel and vacuum glazing

The development of smart windows must provide low solar heat gain with a low overall heat transfer coefficient, avoid humidity and condensation in cold regions, generate clean electricity, and admit comfortable levels of daylight. Therefore, methods for integrating semi-transparent (or 50.8% transparent) CdTe solar cell strings-based glazing with structured-cored mesh translucent vacuum insulationpanels and indium sealed vacuum glazing are described for modernizing smart windows. This study reports experimental and theoretical studies on the thermal and electrical performances of six different glazing systems. These systems include semi-transparent photovoltaic glazing (GPV), vacuum glazing (VG), translucent vacuum insulation panel (GVIP), semi-transparent PV with VG (VGPV), and semi-transparent PV with translucent vacuum insulation panel (VIPPV), and their performances will be compared with that seen with single glazing (SG). These glazing systems are designed, constructed, and tested using a hot box calorimeter, and with and without the effects of simulated indoor solar radiation. The center-of-pane U-values, the transient temperature variations of the inner and outer surfaces of the glazing systems, the open circuit voltages, the short circuit currents, the fill factors, and the steady-state temperature contours were determined. For the first time, the moisture condensation pattern is also depicted for these systems and will be of value for applications in harsh, cold regions. A 3D finite-volume heat transfer model is developed and validated with the experimental results, allowing comparison of the thermal performances of these glazing systems under ASTM boundary conditions. The results showed that the VGPV system achieved a lower U-value than did the VIPPV system. The steady-state center-of-pane temperature differences seen with a solar irradiation level of 1000 W·m−2 are 55 °C, 32.5 °C and 5 °C for the VGPV, VIPPV, and GPV systems, respectively. The validated center-of-pane U-values for the VG, VGPV, VIPPV, and GPV systems, each with dimensions of 15 cm × 15 cm, are predicted to be 1.3, 1.2, 1.8, and 6.1 W·m-2K−1, respectively. The results also show that the use of either the VGPV or VG systems eliminates moisture condensation. It is concluded that VGPV and VIPPV generate comparatively less power but provide higher thermal insulation.

Water flow boiling in a partially modified microgap with shortened micro pin fins

The flow boiling in microgap is attracting much attention due to the large potential for directly cooling the heat sources of the electronic devices, which has many advantages like high heat dissipation rate, easy fabrication, and less working fluid consumption while supplying a large cooling area, etc. However, the local dryout easily occurs near the outlet of the microgap due to the continuous evaporation of the liquid film surrounding the large confined bubble and the late rewetting at downstream, which reduces the heat transfer performance at some operating conditions. In this study, a novel configuration of microgap was proposed in which an array of shortened micro pin fins was partially machined at downstream. Flow boiling experiments were performed in the microgap using deionized water with the inlet temperature of 65 °C. The effects of heat flux and mass flux on heat transfer performance and pressure drop characteristics during flow boiling were tested at different mass fluxes ranging from 261 to 1861 kg/m2s for effective heat flux varying from 487.3 to 4738.5 kW/m2. High speed visualization was conducted to observe the flow pattern transitions and explore the heat transfer mechanisms of flow boiling in the present microgap. Experimental results showed that the flow reversal was not observed over the entire operating range. In addition, the local heat transfer coefficient in the fin region was larger than that in the smooth surface region, which was attributed to the simultaneous enhancement of the shortened micro pin fins on nucleate boiling and convection evaporation in this region. The function of storing liquid and suppressing local dryout of the shortened micro pin fins was verified in this study.

Numerical aero-thermal study of high-pressure turbine nozzle guide vane: Effects of inflow conditions

Accurate predictability of high-pressure turbine nozzle guide vane aero-thermal performance is highly desired in the development campaign due to the exposure of the component to a frequent and high heat load. In this paper, the representative vane profile in modern aero-engines is numerically studied. Aerodynamics and aero-thermal validations of the blade profile have been performed in comparison with the available experimental data. It has been shown that a satisfactory agreement could be achieved with the use of the transitional turbulence model shear stress transport γ–θ due to its superiority in capturing the laminar–turbulent transition. Sensitivity studies on the increase in the inlet turbulence intensity, inlet endwall boundary layer thickness, and inlet total temperature profile have been performed to understand the impact of inflow conditions’ uncertainty on the aero-thermal predictability. Increasing the inlet turbulence intensity increases the pressure surface heat transfer coefficient and induces an earlier transition onset on the suction surface. Due to the rapid decay of turbulence intensity in the numerical model, the use of an artificially high inlet turbulence intensity has been shown to be effective in the prediction improvement. On the other hand, the change in the inlet boundary layer thickness influences the formation and strength of the secondary flow, namely, horseshoe vortex and passage vortex. These secondary flow phenomena affect the local blade surface heat transfer coefficient in the near-endwall region although the most significant rise in heat transfer is found on the endwall. The temperature distortion amplitude of a hot streak and its relative clocking position with the vane significantly affect the heat flux distribution. In contrast, the heat transfer coefficient is less sensitive to the change in hot streak conditions. However, it has been shown that increasing the temperature distortion amplitude could induce a larger difference among different clocking configurations. In addition, decreasing the difference between the fluid and wall temperature would delay the transition onset and stabilize the boundary layer. Further analysis of the unsteady effects has been carried out by comparing the steady and time-averaged flow solutions. It has been observed that the discrepancy between these solutions is attributed to the flow field nonlinearity. Thus, a significant discrepancy can be found in the laminar–turbulent transition as well as in the trailing edge region. However, since the contribution of these regions on the total area-averaged heat transfer is small, their influence on the total vane heat transfer is limited.

Pressure drop and heat transfer of flow boiling R134a in a dimpled flat duct

A test facility has been built up to investigate the pressure drop and heat transfer of flow boiling R134a in a dimpled flat duct. The experiments are conducted at mass flux from 102.2 to 205.0 kg m−2 s−1, heat flux from 2.3 to 9.3 kW m−2, pressure from 349.8 to 488.4 kPa and vapor quality from 0.05 to 0.95. The results show that the frictional pressure gradient increases with increasing vapor quality and mass flux, while decreases with a rise in pressure. The heat flux has little impact on the frictional pressure gradient. The flow boiling heat transfer coefficient decreases slightly first and then increases significantly with an increase in vapor quality, which is attributed to the combined contributions of nucleate boiling heat transfer and convective evaporation heat transfer. No dryout has been observed in the tested vapor quality region. The flow boiling heat transfer coefficient visibly increases with increasing mass flux and heat flux, while the rise in pressure just slightly increases the flow boiling heat transfer coefficient in low vapor quality region only. The comparisons with correlations for regular channels show that the dimpled flat duct has 3.9 to 8.3 times frictional pressure gradient and 1.3 to 3.0 times heat transfer coefficient.

Heat transfer in rod bundles cooled by supercritical water – Experimental data and correlations

The paper presents the results of a comparative evaluation of existing correlations for the prediction of heat transfer to supercritical pressure water against experimental data for bundles of heated rods. It is shown that the method, developed by the authors for the engineering calculation of the heat transfer coefficient in the supercritical region of the coolant parameters, makes it possible to describe the experimental data for smooth rod bundles with an error of less than 15%. In the case of rod bundles with wrapped wire, the same method gives values of the heat transfer coefficient, which are on average about 8% lower compared to the experimental data obtained. This is connected with the intensification of heat transfer due to swirling the flow and can be considered as a margin when estimating the limiting temperature of the heat transfer wall. Based on the comparison of the calculation results by different methods, the problems of choosing the ratio for determining the heat transfer coefficient in fuel assemblies of reactors with supercritical pressure water are discussed.

Determination of local heat transfer coefficients at the entrance region of streamlined bodies

Prandtl’s two-layer model of the turbulent boundary layer is considered and the expression obtained through the use of the model is applied to calculate the heat transfer coefficient, calculations for which agree well with experimental data on mean values of the coefficients for various bodies. Determination of parameters of this expression is shown for the case of calculating local heat transfer coefficients in the entrance regions of the channels. The main parameters are dynamic velocity, dimensionless thickness of the boundary layer and dimensionless thickness of the viscous sublayer. Based on the power-law and logarithmic velocity profiles, expressions are obtained for calculating the dimensionless parameters of the turbulent boundary layer. A satisfactory agreement of the results of calculations of loc al heat transfer coefficients for the flow over a flat plate and the pipe flow is shown. The presented approach represents a theoretical basis for modeling the local heat transfer for bodies of more complex shapes, if the friction coefficients are known.