Surface Heat Transfer Coefficient Research Articles

Quantifying Heat Transfer Characteristics of the Kroll Reactor in Titanium Sponge Production

Various reactors with different sizes have been widely used for the production of titanium sponge in the Kroll process. But the further commercialization of the forced heat transfer design of the Kroll reactor is limited by lack of standard parameter to evaluate its convective heat transfer characteristics. This work proposes to evaluate and compare the Kroll reactor with the dimensionless Nusselt number. The results shown that the heat transfer coefficients for both surfaces increase with the volume flow rate of cooling air for each dimensionless temperature, and the heat transfer coefficients of the external surface of the reactor are higher than that of the internal surface of the heater. And new correlations regarding the Nusselt number between the cooling air and the external surface of the reactor or the internal surface of the heater are obtained based on experimental data, while the characteristics of the cooling air, equipment and operation parameters are considered.

Experiments and modeling of boiling heat transfer on hybrid-wettability surfaces

This paper studies pool boiling heat transfer on the hybrid-wettability surfaces, i.e., hydrophobic dot/stripe patterns fabricated on a superhydrophilic substrate. It is shown that the nucleate boiling heat transfer coefficient for the hybrid surface is improved compared to both the substrate and plain copper reference, but the critical heat flux (CHF) on the enhanced surface is very complex. The pattern-to-substrate contact angle difference is also concerned: CHF for the hybrid surface increases remarkably with increasing the contact angle difference, but the nucleate boiling heat transfer coefficient declines. Combining the results for both dot and stripe patterns, it is revealed that CHF on the hybrid surfaces is closely associated with the pattern-to-surface area ratio and the pattern-to-pattern spacing: it declines generally with increasing the area ratio; its dependence on the pattern spacing is minor at large area ratios; however, at small area ratios, the pattern spacing plays an increasingly important role because it dominates both vapor-liquid instabilities and surface rewetting, and the optimal pattern spacing for maximal pool boiling heat transfer enhancement can be estimated using the capillary length. A modified theoretical model is proposed to predicting CHF on both homogenous- and hybrid-wettability surfaces.

Flow dynamics over a heated cylinder subjected to high temperature ratios

Unheated, and heated flows with forced convection over a fixed circular cylinder are studied numerically. Reynolds numbers of 80, 100, 150, and 200 are considered at different temperature ratios (T∗) of 1.66, 2.33, and 3. Different thermophysical property models of the fluid were used and implemented. The aerodynamic forces occurring on the cylinder as vortex shedding took place were explored. The drag coefficient, lift coefficient, and Strouhal number are presented and discussed. Moreover, the surface heat transfer coefficient is analyzed for the different cases of heating. It was found that increasing the temperature ratio of the cylinder caused an increase in the static load and a decrease in the dynamic load on the cylinder. In addition, the effects of cylinder surface temperature on the vorticity contours are shown for different Reynolds numbers. It was observed that increasing the temperature ratio of the cylinder caused the vortices in the wake to stretch and become weaker. This leads to total suppression of vortex shedding if sufficient heating is provided. Moreover, while the surface heat transfer coefficient of the cylinder was observed to increase with the increase of Reynolds number at constant temperature ratio, increasing the temperature ratio of the cylinder at a constant Reynolds number reduces the heat transfer coefficient and suppresses the variation in between the upper and lower sides of the cylinder.

Optimization of tube arrangement and phase change material for enhanced performance of solar air heater- A numerical analysis

Application of Solar Air Heater (SAH) could be extended to off sunshine hours by integrating SAH with a Thermal Energy Storage System (TESS). Design optimization of TESS will enhance thermal performance of SAH integrated with TESS. Mass flow rate and inlet temperature of air, type of Phase Change Material (PCM), alignment and dimensions of tubes in TESS play an important role in its design optimization. In the present study, numerical analysis using ANSYS FLUENT was carried out to optimize the design of TESS. Mass flow rate of air in terms of velocity at the inlet was changed from 3.5 m/s to 5.5 m/s in a step of 0.5 m/s. Three different PCMs including paraffin wax, n-octadecane, calciumchloride hexahydrate (CaCl2•6H2O) were considered. Inline, staggered and circular arrangement of tubes were considered in the TESS. Temperature of air at inlet to TESS is changed to 40 °C, 50 °C, 60 °C, 75 °C, 100 °C and 125 °C. Results are obtained in terms of average tube wall temperature, average PCM temperature in tubes, average surface heat transfer coefficient and average heat absorbed by PCM. Tubes in staggered arrangement increased heat absorbed by PCM by 10.45% and 48.03% respectively with paraffin wax and CaCl2•6H2O as PCM compared to tubes in inline arrangement. There is insignificant improvement (0.09%) between staggered and inline grid arrangements when n-octadecane was used as PCM. Tubes in circular arrangement increased heat absorbed by PCM by 9.17%, 1.55% and 1.08% respectively with paraffin wax, n-octadecane and CaCl2•6H2O as PCM compared to tubes in staggered arrangement. Similarly, it increased heat absorbed by PCM by 17.89%, 1.63% and 48.59% respectively compared to tubes in inline arrangement.

Investigating of the Heat Transfer Characteristics of Impinging Flow on a Downward-Facing Surface

Critical heat flux (CHF) is an important heat transfer deterioration phenomenon during boiling heat transfer. It has been extensively studied, especially in the field of nuclear energy. Previous research has found that downward-facing heat transfer is worse than upward-facing and should be paid more attention. In this paper, the boiling heat transfer process under different flow rates and inlet distances is investigated. Seven experimental cases were made including a pool boiling case. The experiment studied the effect of inlet distances under small flow rates, which is not covered by previous research. Analysis of the CHF mechanism included surface temperature curves, boiling curves, bubble behaviors, and heat transfer coefficient. The fluctuation of the surface temperature of forced convention cases was observed due to the bubbles sliding along the heating surface. The phenomenon of vapor film fragmentation could also be found. The results show that even at small flow rates, CHF occurring time is postponed and the CHF value increases. Reducing inlet distance or increasing flow rate can both promote boiling heat transfer, thereby enhancing CHF.

Computational Investigation on Natural Convection Heat Transfer in Vertical Plate Fin Arrays

Objectives: To perform computational studies on different combinations of plate fin arrays of different shapes for studying their performance in natural convection heat transfer. To compare the heat transfer performance of uniform notched fins with hybrid notched fins and also performance of normal notched fins with inverted notched fins. Methods: SolidWorks Flow Simulation software is used for the present computational studies. Three types of vertical plate fin arrays viz. plain rectangular fin, rectangular fin with square notch and rectangular fin with semi-circular notch are used with their seven combinations. These fin array combinations are: plain rectangular, rectangular square notched, rectangular semi-circular notched, rectangular inverted square notched, rectangular inverted semi-circular notched, hybrid rectangular square-semi-circular notched and hybrid rectangular inverted square-semi-circular notched. Findings: The highest average heat transfer coefficient of 7.82 W/m2K is found to be for inverted hybrid square-semicircular notched fin array. This value of heat transfer coefficient is about 8% higher than compared to that with plain rectangular plate fin array (7.25 W/m2K) subjected to same operating conditions. Average heat transfer coefficients for inverted plate fin array combinations are high as compared with the non-inverted combinations. Also, hybrid plate fin arrays, whether inverted or not, show higher heat transfer coefficient values as compared with uniform notched fin arrays. Novelty : Seven types of plate fin array combinations are studied computationally. SolidWorks Flow Simulation software is used very effectively for this 3D CFD analysis. The behaviour of fluid i.e. air during natural convection flow is studied using air flow velocity trajectories. Surface contours are effectively used for studying variation in surface temperature and heat transfer coefficient at all locations of fin arrays. Keywords Natural convection, plate fin array, heat transfer coefficient, CFD, heat transfer enhancement

METHOD ACHIEVING COOLING UNIFORMITY AND OPTIMAL PLATE-CURVATURE DURING ULTRA-FAST COOLING IN A HSM

The flow field in the top and bottom surface of the hot rolled strip is different during cooling process with effect of gravity. Then it can affect the strip cooling uniformity of the top and bottom surface, and the plate curvature problems may be appeared. The finite element method was taken to study the plate curvature affecting law and a conclusion was obtained: the uniformity of the heat transfer coefficient in the top and bottom surface was the key to keep plate curvature well after rolling. The finite volume method was taken to calculate the heat transfer coefficient during run-out table laminar cooling (LC) and ultra-fast cooling (UFC) with different top nozzle fluxes and water flux ratios. The heat transfer coefficient and its distribution with different cooling methods and process parameters were obtained, and some conclusions were obtained by analysis: the bottom and top surface heat transfer coefficient can be kept nearly the same by adjusting water flux ratio between the bottom nozzle and top nozzle. The optimal water flux ratios of laminar cooling were 1.20 and 1.15 when top nozzle fluxes were 100m3/h and 120m3/h respectively. The optimal water flux ratios of ultra fast cooling were 1.08, 1.10, 1.15, 1.20 and 1.20 when top nozzle fluxes were 80m3/h, 100m3/h, 120m3/h, 140m3/h and 160m3/h respectively. The obtained results and water flux ratio calculating model were used in several strip cooling lines of the hot strip mill lines and obtained favorable effect.

Pool Boiling of Water on Surfaces with Open Microchannels

Boiling, as the most efficient type of convective heat transfer, is an area of interest in many fields of industry and science. Many works have focused on improving the heat transfer efficiency of boiling by altering the physical and chemical properties of surfaces by using different technological processes in their fabrication. This paper presents experimental investigations into pool boiling on enhanced surfaces with open microchannels. The material of the fabricated surface was copper. Parallel microchannels made by machining were about 0.2, 0.3, and 0.4 mm wide, 0.2 to 0.5 mm deep, and spaced with a pitch equal to twice the width of the microchannel. The experiments were carried out in water at atmospheric pressure. The experimental results obtained showed an increase in the heat flux and the heat transfer coefficient for surfaces with microchannels. The maximum (critical) heat flux was 2188 kW/m2, and the heat transfer coefficient was 392 kW/m2K. An improvement in the maximum heat flux of more than 245% and 2.5–4.9 times higher heat transfer coefficient was obtained for the heat flux range of 992–2188 kW/m2 compared to the smooth surface. Bubble formation and growth cycle in the microchannel were presented. Two static computational models were proposed to determine the bubble departure diameter.

Quality Dahi Preparation in Automated Controlled Ambient Conditions

This automated controlled system eliminates the need of two separate places for incubation of the curd at higher temperature in the range of 39 to 43°C and then shifting the set curd cups into cold rooms for storage purpose and kept at about 4 to 5°C.The transient cooling process of dahi, set in cups and also in steel containers placed in cold air flow was conducted in this experimental study. Recording of the temperature of dahi-cups and of the supplied air were done at regular intervals until the temperature reached below 4°C to 5°C. The exponential curves of cooling by forced convection process for the dimensionless temperature of curd were obtained for the trials conducted at the different velocities of cooling air flow. It was found that the surface heat transfer coefficient increased and duration of cooling decreased by increasing the air velocities from 0.5 m/s by evaluating the Biot number and surface heat transfer coefficient. In the initial period of cooling this pattern was more effective and reduced for higher velocities of air from 3.5 to 4.5 m/s. The method of incubation and storing dahi-cups at the same place and changing the ambient temperature of the whole environment instead of changing the place of dahi-cups for cooling purpose have been applied in this research work to control the problem of whey-off in set-curd.

Performance Comparison of Microchannel Heat Sink Using Boron-Based Ceramic Materials

Microchannel heat sink plays a vital role in removing a considerable amount of heat flux from a small surface area from different electronic devices. In recent times, the rapid development of electronic devices requires the improvement of these heat sinks to a greater extent. In this aspect, the selection of appropriate substrate materials of the heat sinks is of vital importance. In this paper, three boron-based ultra-high temperature ceramic materials (ZrB2, TiB2, and HfB2) are compared as a substrate material for the microchannel heat sink using a numerical approach. The fluid flow and heat transfer are analyzed using the finite volume method. The results showed that the maximum temperature of the heat source didn’t exceed 355K at 3.6MWm-2 for any material. The results also indicated HfB2 and TiB2 to be more useful as a substrate material than ZrB2. By applying 3.6 MWm-2 heat flux at the source, the maximum obtained surface heat transfer coefficient was 175.2 KWm-2K-1 in a heat sink having substrate material HfB2.

AN EXPERIMENTALLY VALIDATED LOW ORDER MODEL OF THE THERMAL RESPONSE OF DOUBLE-WALL EFFUSION COOLING SYSTEMS FOR HP TURBINE BLADES

Abstract Transpiration cooling represents the pinnacle of turbine cooling and is characterised by an intrinsic porosity achieving high internal convective cooling, and full coverage film cooling. The quasi-transpiration, double-wall effusion system attempts to replicate the cooling effect of transpiration cooling. The system is characterised by a large wetted area providing high internal convective cooling performance, with a highly porous external wall allowing the formation of a protective cooling film. This paper presents a low-order thermal model of a double-wall system designed to rapidly ascertain cooling performance based solely on the geometry, thermal conductivity, and approximate surface heat transfer coefficients. Initially validation uses experimental data with heat transfer coefficients for the low order model obtained from fully conjugate CFD simulations. A more controlled CFD study is then undertaken with both fully conjugate and fluid only simulations performed on several double-wall geometries to ascertain both overall and film effectiveness data. Data from these simulations are used as inputs to the low order thermal model and the results compared. The low order model successfully captures both the trends and absolute cooling effectiveness achieved by the various double-wall geometries. The model therefore provides a powerful tool whereby the cooling performance of double-wall geometries can be near instantaneously predicted during the initial design stage, potentially allowing geometry optimisation to rapidly occur prior to more in-depth, costly and time-consuming analyses. This benefit is demonstrated via the implementation of the model with input boundary conditions obtained using empirical correlations.

Predicting unsteady heat transfer effect of vehicle thermal management system using steady velocity equivalent method.

In the process of vehicle development, the unsteady simulation of thermal management system is very important. A 3D-CFD calculation model of vehicle thermal management is established, and simulations were undertaken for uphill with full loads operations condition. The steady results show that the surface heat transfer coefficient increases to the quadratic parabolic relationship. The unsteady results show that the pulsating temperatures of exhaust and external airflow are higher than about 50°C and lower than 10°C, respectively, and the heat dissipating capacities are higher than about 11%. Accordingly, the conversion equivalent exhaust velocity increased by 1.67%, and the temperature distribution trend is basically the same as unsteady results. The comparison results show that the difference in the under-hood should be not noted, and that the predicted exhaust system surface temperatures using steady velocity equivalent method are low less 10°C than the unsteady results. These results show the steady velocity equivalent method can be used to predict the unsteady heat transfer effect of vehicle thermal management system, and the results obtained by this method are basically consistent with the unsteady results. It will greatly save computing resources and shorten the cycle in the early development of the vehicle thermal management system.

Experimental study on temperature drop during roller quenching process of large-section ultra-heavy steel plate.

Large-section ultra-heavy steel plates feature low cooling rate of central part during quenching process, as well as great differences in texture and performance along thickness direction. Wet jet impingement quenching test apparatus and multi-channel temperature recorder were developed to explore the temperature drop characteristics and its influential factors of ultra-heavy steel plate. The temperature drop curves of plates with different thickness (160, 220, and 300 mm) and weight (23-43 tons) were recorded during the quenching process, accompanied with the exploration on distribution rules of temperature gradient, heat flux density and cooling rate along thickness direction under different water volumes (9000-9400 m3/h) and pressures (0.4-0.8 MPa), upon which the three-dimensional inverse heat conduction model, surface heat transfer coefficient model and thermophysical parameter model were established using finite element and optimization methods. The overall results indicate the symmetric heat transfer between the upper and lower plate surfaces can be realized, when the water flow ratios of the upper and lower surfaces were 1:1.25 and 1:1.4 under pressures of 0.8 MPa and 0.4 MPa, respectively. The "temperature return" phenomenon was accompanied with changing heat flux density of wall surface of 220 mm- and 300 mm-thick plates. The lowest cooling rate along thickness direction appeared at 1/4 thickness position for the 160 mm- and 220 mm- thick plates during quenching, which was attributable to the synergy between heat flux density and temperature gradient. This work may pave a way for improving the cooling rate and quenching uniformity of ultra-heavy steel plates along thickness direction.

On-site experimental study on fouling and heat transfer characteristics of flue gas heat exchanger for waste heat recovery

A large number of fly ash particles in flue gas easily cause the fouling problem of heat exchangers, which seriously affects the efficient utilization of flue gas waste heat. In this study, experimental and numerical investigations on fouling and heat transfer characteristics in cross flow heat exchangers were carried out. The on-site experimental system was established, and the heat transfer coefficients for different heat-transfer surfaces at the fouling state were monitored in real time. The weakened degree of heat transfer coefficient and fouling thermal resistance are adopted to assess the effect of fouling on the heat transfer performance and compare the practicality of four different heat-transfer surfaces under dusty conditions. Numerical investigation was performed with the fouling model established in previous work to model the thermal-hydraulic and fouling processes. Numerical results are compared with experimental measurements and a good agreement is achieved. Results showed that compared with the aligned circular tube bundle, using H-type fins or honeycomb tube bundle both improve the anti-fouling performance, especially for the honeycomb H-type finned tube bundle, which has the lowest fouling thermal resistance. Therefore, due to the characteristics of easily soot-blowing and lower fouling thermal resistance, honeycomb heat exchangers are suitable for waste heat utilization of dusty flue gas.

Influence of Active Cooling at the Trailing Edge on the Thermal Behavior of a Tilting-Pad Journal Bearing

In tilting-pad journal bearings (TPJB) with a non-flooded lubrication concept, higher maximum pad temperatures occur than with a flooded bearing design due to the lower convective heat transfer at the pad edges. In this paper, we present an approach to influence the thermal behavior of a five-pad TPJB by active cooling. The aim of this research is to investigate the influence of additional oil supply grooves at the trailing edge of the two loaded pads on the maximum pad temperature of a large TPJB in non-flooded design. We carry out experimental and numerical investigations for a redesigned test bearing. Within the experimental analysis, the reduction in pad temperature is quantified. A simulation model of the bearing is synthesized with respect to the additional oil supply grooves. The simulation results are compared with the experimental data to derive heat transfer coefficients for the pad surfaces. The experimental results indicate a considerable reduction of the maximum pad temperatures. An overall lower temperature level is observed for the rear pad in circumferential direction (pad 4). The authors attribute this effect by a cooling oil carry-over from the previous pad (3). Within the model limits, a good agreement of the simulation and experimental results can be found.

Numerical Analysis of quenching and cooling process of Cr12MoV Die Steel under Thermo-mechanical Coupling condition

Cr12MoV die steel has the characteristics of good wear resistance, small quenching deformation, high hardenability and hardness, so it requires high heat treatment process. With the emergence of computer simulation technology, it is convenient for the evolution and calculation of quenching process. In this paper, by using the technical route of combining nonlinear surface heat transfer coefficient with finite element simulation, the thermo-mechanical coupling theory is applied to the quenching process of Cr12MoV steel, and the temperature field and stress field within 300s before quenching are studied with the help of ABAQUS finite element software. The results show that the node temperature decreases with the increase of quenching time, the lowest temperature at edges and corners is about 21°C, the heat flux vector increases with the increase of quenching time, the yield stress increases with the increase of quenching time, and the maximum corner stress is about 751MPa. The experimental results of this paper are beneficial to the evolution calculation of material quenching process and the optimization of heat treatment method.

A Three-Dimensional Analysis of Homogeneous and Functionally Graded Thermoelectric Cylinders

This paper analyzes a homogeneous thermoelectric (TE) circular cylinder as well as an exponentially graded TE cylinder under three-dimensional (3D) conditions. Analytical solutions of the temperature fields are obtained using an eigenfunction expansion method. The energy conversion efficiencies are determined and the material property gradation and leg geometry effects are examined. For the homogeneous TE material, it is found that the efficiency of the TE cylinder is almost the same as that of the cuboid TE leg with the side 2 W equal to the diameter 2R of the cylindrical leg. The efficiency of the cylindrical leg, however, is slightly higher than that of the cuboid leg if the two legs have the same length and cross-sectional area. In this case, the difference in energy efficiency between the two leg geometries depends on the ratio of the lateral dimension to the leg length and vanishes when the lateral dimension goes to infinity. For the exponentially graded TE cylinder, the numerical examples show that the efficiency is increased by appropriate material property gradations but is decreased by the 3D effects. Finally, the analytical solution indicates that besides the figure of merit, the two dimensionless parameters R/L and hL/k, where L is the leg length, h is the surface heat transfer coefficient, and k is the thermal conductivity, are also important factors that influence the energy efficiency of TE cylinders in 3D.

Heat Transfer in Highly Turbulent Separated Flows: A Review

The study of flows with a high degree of turbulence in boundary layers, near-wall jets, gas curtains, separated flows behind various obstacles, as well as during combustion is of great importance for increasing energy efficiency of the flow around various elements in the ducts of gas-dynamic installations. This paper gives some general characteristics of experimental work on the study of friction and heat transfer on a smooth surface, in near-wall jets, and gas curtains under conditions of increased free-stream turbulence. Taking into account the significant effect of high external turbulence on dynamics and heat transfer of separated flows, a similar effect on the flow behind various obstacles is analyzed. First of all, the classical cases of flow separation behind a single backward-facing step and a rib are considered. Then, more complex cases of the flow around a rib oriented at different angles to the flow are analyzed, as well as a system of ribs and a transverse trench with straight and inclined walls in a turbulent flow around them. The features of separated flow in a turbulized stream around a cylinder, leading to an increase in the width of the vortex wake, frequency of vortex separation, and increase in the average heat transfer coefficient are analyzed. The experimental results of the author are compared with data of other researchers. The structure of separated flow at high turbulence—characteristic dimensions of the separation region, parameters of the mixing layer, and pressure distribution—are compared with the conditions of low-turbulent flow. Much attention is paid to thermal characteristics: temperature profiles across the shear layer, temperature distributions over the surface, and local and average heat transfer coefficients. It is shown that external turbulence has a much stronger effect on the separated flow than on the boundary layer on a flat surface. For separated flows, its intensifying effect on heat transfer is more pronounced behind a rib than behind a step. The factor of heat transfer intensification by external turbulence is most pronounced in the transverse cavity and in the system of ribs.

Thermal imaging method for quantitative and qualitative assessment of the acceptable heat losses in configurations of structures in the diagnosis of buildings

The article discusses the possibility of using a hardware thermometric diagnostic method based on a thermal imaging device in energy-intensive low-rise industries. The proposed optoelectronic device, a thermal imager, can operate in real time, store and process information, and transmit the received data to the consumer. The solution, the creation of a model - an algorithm with software, can be used for the purpose of obtaining numerical values of heat losses and the accompanying characteristics of differentiated temperature states with the surface heat transfer coefficient.

Heat transfer enhancement of laminar impinging slot jets by nanofluids and metal foams

A study on impinging slot jets in a confined channel filled with metal foam, saturated with nanofluids, and lower wall partially heated is numerically accomplished. The partially heated wall is at an assigned temperature and buoyancy effects are taken into consideration. A two-dimensional domain is analyzed considering the local thermal non-equilibrium for the metal foam and the single-phase model is assumed to simulate the nanofluids at assigned Rayleigh number for several Peclet numbers. The governing equation solutions are accomplished using the Ansys-Fluent code and simulations are carried out for aluminum foams with nanofluids, Al2O3/water based, as working fluids. Several porosity values, from 0.90 to about 0.97, for different pore density, pore per inches (PPI), from 5, to 40 PPI, are used. The volumetric concentrations change from 0% to 5% with a particle diameter of 30 nm. The distance between the slot jet outlet section and the lower heated surface is five times the slot jet width. The presented results point out that the dimensionless convective heat transfer coefficients increase with increase in values of Peclet number, and decrease with the volumetric concentration. It is noted that the surface heat transfer coefficient shows different behavior varying porosity for assigned Peclet and Rayleigh numbers. Some correlations for the average total Nusselt number in terms of porosity and volumetric concentration are proposed in quadratic forms for assigned pore density and Peclet number.