Variation Of Heat Transfer Coefficient Research Articles

Investigation of the effect of blockage ratio on flow and heat transfer in the wake region of a cylinder embedded in a channel using whole field dynamic measurements

Non-intrusive field measurements in the wake region of a circular adiabatic cylinder fitted in a channel are reported. The top wall of the channel is subjected to constant heat flux while maintaining the bottom wall and the cylinder under adiabatic condition. The cylinder diameter has been varied to achieve three different blockage ratios (D/H) of 0.25, 0.38 and 0.5. Experiments are performed in the range of 63≤Re ≤ 165 with water as the coolant fluid under hydrodynamically fully developed and thermally developing conditions. Effect of blockage ratio on heat transfer enhancement capability, its capacity to alter the flow physics by modifying the size of vortices, the downstream distance they travel and the Reynolds number at which the vortices get initiated, is reported. Path-integrated temperature distribution in the channel has been mapped through the classical form of Mach Zehnder interferometer. The temperature distribution thus obtained has been used to determine the variations in local heat transfer coefficient along the length of the top horizontal surface of the channel. Schlieren deflectometry is used to quantify the vortex shedding frequency under three different blockage ratio regimes. The experimentally obtained results are benchmarked and validated by performing numerical simulations using FLUENT. It has been observed that a decrease in blockage ratio at a constant Re leads to alterations in the size of the generated vortices, which manifest as an increase in the vortex shedding frequency. Vortex shedding is seen to get initiated at relatively lower value of Re (= 65) when the blockage ratio is low (D/H = 0.25). However, when the wake transition to three-dimensionalities is considered, an increase in blockage ratio from D/H = 0.25 to 0.5 leads to an increase in the value of transition Re (165 for D/H = 0.5). It is observed that the vortices travel maximum downstream distance for blockage ratio of 0.38. For all blockage ratios, due to vortex shedding, the thermal boundary layer assumes a corrugated shape and aids in enhancing the heat transfer rates from the channel surface.

Comparative Study of Heat Transfer Performance and Visualization Images of Superfluid Helium Boiling in Narrow Two-dimensional channel

In a narrow channel, boiling heat transfer in He II is much different from that in the case of an open bath. Even under saturated pressure condition, rapid temperature rise due to the onset of film boiling is not seen when the temperature reaches the lambda temperature, but only the temperature oscillation is detected. The behaviour of this state looks like the nucleate boiling, though it is known the steady nucleate boiling state is absent in He II in open bath. The high heat transfer coefficient in this quasi-nucleation boiling state above the lambda temperature suggests the range where this advantage in heat transfer appears is wider than that based on simple prediction of He II heat transfer. The appearance of metastable states of superheated He II and superheated He I should be considered in the case of a narrow channel. In this study, the visualization experiment result using a transparent heater and the transient temperature measurement data on a copper heater surface had been compared with each other. In the case of rather high heat flux, several film boiling modes were observed in a narrow two-dimensional channel. The peak of the heat transfer coefficient appeared around the lambda pressure. In the case of low heat flux, the heat transfer coefficient variation with the pressure shows sudden rise around the lambda point. The heat transfer coefficient under nearly saturated pressure condition below the lambda pressure is larger than that above the lambda pressure.

PII: S0928-1258(20)30052-8

In this study, the falling film evaporation heat transfers of R290 on an array composed of five enhancement horizontal tubes (groove tubes) are studied. The tests are performed at constant saturation temperatures 5.5 °C with change of heat flux from 10 to 40 kW/m2. The film Reynolds number ranges from 200 to 2200 and the film flow rate of refrigerant is between 100 and 660 kg/h.The results show that the film flow rate and heat flux have significant effects on R290 falling film evaporation heat transfer coefficients (HTCs) of the tubes in the tube array. With decreasing the film flow rate on the five tubes the tube HTCs display two stages, a plateau stage and a sharp drop stage. The heat transfer coefficients firstly keep more or less constant at the plateau stage and then decreasing rapidly. At the same nominal film Reynolds number the tube averaged heat transfer coefficients of tube No. 1 to tube No. 5 decrease in order of the increasing tube number from top to bottom of the array. The falling film evaporation HTC of R290 on single enhanced tube is about 4.5 times of single smooth tube HTC, and the HTCs of the enhanced tube array is higher than single smooth tube by more than 2.5 times. In addition, the R290 HTCs of the tube array are higher than those of R134a for the same tube array in the plateau region by about 25%. It is found that at high heat flux of 30–40 kW/m2, the heat transfer coefficient variation with film Reynolds number of the lower enhanced tubes in the tube array exhibits severe undulating characteristics.

Development of correlation for natural convection heat transfer for large horizontal calandria vessel for in-calandria retention of corium in PHWRs

The natural convection heat transfer from the outer surface of horizontal heated calandria vessel (CV) of PHWR which is submerged in confined vault water is studied. For this, experiments were carried out in a scaled test setup. Experimental results were used to estimate the heat flux and heat transfer coefficient around CV. Further these experimental results are also used to develop two correlations. First one is the estimation of angular variation of local natural convection heat transfer coefficient in terms of the modified Rayleigh number with angle from the bottom of the cylinder CV. Another one is for estimation of angular variation of local natural convection heat transfer coefficient once averaged heat transfer coefficient is known.

Heat and Mass Transfer in Hydromagnetic Second-Grade Fluid Past a Porous Inclined Cylinder under the Effects of Thermal Dissipation, Diffusion and Radiative Heat Flux

Current disquisition is presented to excogitate heat and mass transfer features of second grade fluid flow generated by an inclined cylinder under the appliance of diffusion, radiative heat flux, convective and Joule heating effects. Mathematical modelling containing constitutive expressions by obliging fundamental conservation laws are constructed in the form of partial differential equations. Afterwards, transformations are implemented to convert the attained partial differential system into ordinary differential equations. An implicit finite difference method known as the Keller Box was chosen to extract the solution. The impact of the flow-controlling variables on velocity, temperature and concentration profiles are evaluated through graphical visualizations. Variations in skin friction, heat transfer and mass flux coefficients against primitive variables are manipulated through numerical data. It is inferred from the analysis that velocity of fluid increases for incrementing magnitude of viscoelastic parameter and curvature parameter whereas it reduces for Darcy parameter whereas skin friction coefficient decreases against curvature parameter. Assurance of present work is manifested by constructing comparison with previous published literature.

Falling film evaporation in a triangular tube bundle under the influence of cross vapor stream

Abstract The falling film evaporation and the effect of cross vapor stream in falling film evaporator with triangular tube bundle using R123 was experimentally investigated. The variations of local and bundle average heat transfer coefficients were described. The tube bundle consisted of 4 × 3 (column × row) of triangular horizontal copper tubes. The tests without vapor effect were conducted with nominal heat fluxes of 20 to 60 kWm–2, saturation temperatures of 6 to 16 °C and film flow rates of 0.016 to 0.18 kgm–1s–1. Cross vapor stream effect experiments were operated at three heat fluxes of 20, 30 and 40 kWm–2 and two film flow rates of 0.035 and 0.07 kgm−1s−1, and the vapor velocity at the narrowest interstice in the tube bundle varies from 0 to 5.0 ms−1. It is indicated that the heat transfer is seriously influenced by the bundle effect while less affected by changing of saturation temperature. With the increase in vapor velocity, the heat transfer performance is generally weakened; the cross vapor stream has a strong influence on falling film evaporation of R123.

Modeling and Simulation of Heat Transfer Phenomenon Related to Mold Heating during Investment Casting

Cast shape during investment casting process dictates properties and service life of the casting. These properties are the function of cast part parameters (both static and dynamic), part geometry and hence mold geometry, nature and type of metal being cast, properties (extrinsic and intrinsic) and processing parameters (rate of heating, rate of cooling, rate of pouring). Improper and inadequate manipulation and modification of mold properties degrade the properties and life of casting altogether. A mathematical model is developed using standard transport equations incorporating all heat transfer coefficients (HTCs) to determine the effect of external mold heating on the properties of the final casting and a simulation is performed in C++ to validate it against experimental results. Pure iron is casted in investment molds of silica sand with zircon coating. Airflow near the mold surfaces was partially restricted due to geometry of the molds and arrangement of the pieces around a tree. The variations in heat transfer coefficient contribute towards total heat transfer out of mold surface. External heating is found to be very effective for improved casting properties. The mold heat transfer profile is found to be in good agreement with experimental values validating the effectiveness of mold heating.

Effects of Nanofluid Flow in Micro channel Heat Sink for Forced Convection Cooling of Electronics Device: A Numerical Simulation

A computational study is carried out on a rectangular microchannels heat sink using nanofluids flow for cooling of electronics device under uniform heat flux condition. In the present investigation water, ethylene glycol and a mixture of ethylene glycol (20%wt) and water are considered as base fluids with varying concentration of five different nanoparticles includingAl2O3, TiO2, CuO, SiO2 and ZnO. Numerical computations are performed using ANSYS Fluent software by considering the single phase model and results are validated with available experimental and numerical data. Further parameters like thermal resistance, pumping power, local heat transfer coefficient and temperature variation of IC chip are presented and analysed. It was noted that with addition of nanoparticles there is sharp increment in local heat transfer coefficient and decrements in local thermal resistance compared to base fluid but at same time viscosity of fluid increases that provide more drag or pressure drop which ultimately increases the pumping power. -Water nanofluid of concentration 1% and 4% give large improvement in heat transfer parameters and at the same time there is little enhancement in pressure losses or pumping power also it has less cost and more stability in base fluid as compared to other nanofluids.

Experimental investigation of transient heat transfer coefficient in natural convection with Al2O3-nanofluids

In this manuscript, the heat transfer coefficient of de-ionized water based nanofluid’s containing Al2O3 nanoparticles under natural convection is experimentally investigated. The thermophysical properties of the prepared nanofluid samples which have great roles in the heat transfer process such as the thermal conductivity and viscosity are experimentally measured and their effects are discussed. Furthermore, the transient temperature of the hot and cold walls and the other sides of the container enclosure are measured and the variation of heat transfer coefficient with the volume fraction of the nanoparticles is shown. According to our results, the heat transfer coefficient of nanofluids enhanced at low volume fractions while with further increase in the volume fraction of nanoparticles it decreased as a result of the increase in viscosity. The results show that in comparison with de-ionized water the heat transfer coefficient of nanofluids containing 0.1% volume fraction nanoparticles had a 16.0% enhancement but for a 3.0% volume fraction it had a 24.0% decrease.

Magneto-thermal coupling analysis of the permanent magnet eddy current brake under intensive impact load

To more realistically describe the operation of an eddy current brake under intensive impact load, a magnetic-thermal bidirectional coupling model of the eddy current brake is established considering the existence of temperature. The variation of material and convective heat transfer coefficient with temperature are considered in the model. The iterative method is used to decouple the model. Through the analysis of the numerical simulation results, the temperature distribution and transmission of the brake during operation are obtained. By comparison, it is found that the calculated braking force of the magneto-thermal coupling model is lower than the calculated value without considering the temperature field due to the influence of temperature. And the higher the temperature, the greater the decrease.

Heat Conduction at a Variable Heat-Transfer Coefficient

Some practically important problems of unsteady heat conduction with a time-variable relative heat-transfer coefficient are considered. Various approaches to finding a solution to the analytical problem are systematized: decomposition of the generalized integral Fourier transform, the serial expansion of a sought temperature function, and the reduction of the problem to an integral, second-order Volterra equation. It is demonstrated that the solution is reduced to an infinite series of successive approximations of different functional form in all cases, and the main objective of each of these approaches is to find the most advantageous first approximation. Particular cases of the time dependence of the relative heat-transfer coefficients are considered: linear, exponential, power, and root cases. Analytical solutions and numerical experiments are described, and some specific features of the temperature curves of a number of mentioned dependences are revealed. It is established that the picture of the change in the temperature curve for the linear time law of the heat-transfer coefficient becomes appreciably different from the classic case of a constant coefficient, whereas the exponential dependence does not introduce any essential changes.

Prismatic-core advanced high temperature reactor and thermal energy storage coupled system – A preliminary design

Abstract This study presents an initial design for a novel system consisting in a coupled nuclear reactor and a phase change material-based thermal energy storage (TES) component, which acts as a buffer and regulator of heat transfer between the primary and secondary loops. The goal of this concept is to enhance the capacity factor of nuclear power plants (NPPs) in the case of high integration of renewable energy sources into the electric grid. Hence, this system could support in elevating the economics of NPPs in current competitive markets, especially with subsidized solar and wind energy sources, and relatively low oil and gas prices. Furthermore, utilizing a prismatic-core advanced high temperature reactor (PAHTR) cooled by a molten salt with a high melting point, have the potential in increasing the system efficiency due to its high operating temperature, and providing the baseline requirements for coupling other process heat applications. The present research studies the neutronics and thermal hydraulics (TH) of the PAHTR as well as TH calculations for the TES which consists of 300 blocks with a total heat storage capacity of 150 MWd. SERPENT Monte Carlo and MCNP5 codes carried out the neutronics analysis of the PAHTR which is sized to have a 5-year refueling cycle and rated power of 300 MWth. The PAHTR has 10 metric tons of heavy metal with 19.75 wt% enriched UO2 TRISO fuel, a hot clean excess reactivity and shutdown margin of $33.70 and -$115.68; respectively, negative temperature feedback coefficients, and an axial flux peaking factor of 1.68. Star-CCM + code predicted the correct convective heat transfer coefficient variations for both the reactor and the storage. TH analysis results show that the flow in the primary loop (in the reactor and TES) remains in the developing mixed convection regime while it reaches a fully developed flow in the secondary loop.

Measurement of heat transfer coefficient and pressure drops in a compact heat exchanger with lance and offset fins for water based Al2O3 nano-fluids

Compact Heat Exchangers (CHEs) are widely used in various applications in thermal fluid-systems including aerospace due to their compactness and light weight coupled with high effectiveness. Among the different types of CHEs for Aircraft Environmental Control System (ECS) and Avionics cooling applications, a cross-flow CHE with Lance & Offset fin is of special interest because of their high heat rejection capability. In the present work, the heat transfer coefficient of water based Al2O3 nano-fluid (NF) flowing in a Compact Plate-fin Heat Exchanger having Lance & Offset fins is studied experimentally. As part of experimental investigation a test setup is designed and developed. Experiments are conducted in the range of mass flux from 50 to 350 kg/s m2 (100 ≤ Re ≤ 600) at room temperature as well as at higher temperatures up to 70 °C. In this study, Al2O3 volume concentration in water is varied from 0.255% to 0.51%. The variation of heat transfer coefficients and pressure drops of Al2O3 nano-fluid with respect to water are presented in graphical form for different mass fluxes. The heat transfer coefficient gets enhanced in low NF temperature regions (between 30 °C-45 °C) up to 25% for 1%NF and up to 35% for 2%NF, where as there is only a marginal improvement at higher operating temperatures (between 60 °C-75 °C). The uncertainty analysis is carried out for heat transfer coefficient based on sensors specification.

Experimental and numerical investigation on heat transfer and fluid flow performance of sextant helical baffle heat exchangers

As a promising substitute for the segmental ones, the helical baffles have been gradually popularized in shell-and-tube heat exchangers. Quadrant helical baffles are commonly utilized to induce a pseudo-spiral flow pattern in the shell side of helical baffle heat exchangers, but the triangular leakage loss at the conjunction notches between adjacent baffles has always been the bottleneck. An improved scheme of sextant helical baffle is thus presented as a means of leakage blockage in this work. The experimental approach is implemented and the computational model is established for experimental and numerical analysis of the performance of novel heat exchanger, and a good agreement is achieved by comparing measured data with numerical predictions. The modified flow field characteristic of sextant helical baffle heat exchangers (SHBHXs) is examined in detail in comparison with quadrant helical baffle heat exchangers (QHBHXs). It concludes that the reverse flow in the triangular leakage zone is partly dampened by virtue of special overlapped structure of adjacent sextant helical baffles, and shell-side axial velocity exhibits the feature of uniform distribution. The impact of incline angle is particularly evaluated on variations of shell-side heat transfer coefficient and shell-side pressure drop as well as their ratio, and then the relevant correlations for Nusselt number and friction factor are fitted in the calculation scope. Further comparison results reveal that the thermal performance and the comprehensive performance are much better while the pressure drop is much lower in SHBHXs than in QHBHXs, which can be explicated by a finer field synergy property in SHBHXs on the basis of the field synergy theory. This work can provide theoretical foundation for design and optimization of the new type heat exchanger.

The Design and Validation of a Thermal Boundary Layer Wall Plate

A feedback controlled thermal wall plate designed to investigate thermal boundary layer flows is described and validated. The unique capabilities of the design are the ability to modify the thermal boundary conditions in a variety of ways or to hold the wall-temperature fixed even when the flow above the wall is unsteady and strongly three-dimensional. These capabilities allow for the generation and study of thermal transport in nonequilibrium boundary layer flows driven by different perturbations and of varying complexity. The thermal wall plate and the experimental facility in which the thermal wall plate is installed are first described. The wall-plate is then validated in a zero-pressure-gradient (ZPG) boundary layer flow for conditions of a uniform wall temperature and a temperature step. It is then shown that the wall temperature can be held constant even when a hemisphere body is placed on the wall that produces large localized variations in the convective heat transfer coefficient. Last, since the thermal wall plate is intended to support the study of thermal transport in a variety of nonequilibrium boundary layer flow, several possible experimental configurations are presented and described.

Disturbance modelling through steady-state value deviations: The determination of suitable energy indicators and parameters for energy consumption monitoring in a typical sugar mill

The growing emphasis in alternative revenue streams, from the conversion of sugarcane bagasse into energy co-products has prompted a desire by the sugar factories to achieve higher levels of energy efficiency. This study presents a comprehensive overview of the parameters whose steady-state deviation leads to excess energy use in sugar mills. The effect of eight operating parameters on the defined energy indicators was investigated, using a MATLAB simulation of a 250-tonne per hour sugar mill. The captured energy trends were used to develop energy prediction models based on the parameters whose steady-state offsets resulted in excess energy use. High prediction accuracies of over 95% were obtained when the developed energy prediction models were validated for seasonal cane quality and evaporator heat transfer coefficient variations. Increased A-massecuite recycling had a more pronounced effect on the energy used per kilogramme of water in the evaporator unit as compared to the crystallisation unit. Of the eight parameters, elevated imbibition water use and A-massecuite recycling had a cumulative percentage effect of 79% on the overall steam used per tonne of cane and sugar produced. Hence increasing the syrup concentration, decreasing imbibition water use and massecuite recycling are preferred operational strategies for improved energy-efficiency in a sugar mill. Overall, this study strengthens the existing literature by illustrating an approach for developing suitable energy indicators for excess energy use monitoring in sugarcane mills.

Experimental and computational investigation on two-phase flow and heat transfer of highly subcooled flow boiling in vertical upflow

This study explores subcooled nucleate flow boiling of FC-72 in a rectangular channel having two opposite heating walls at different mass velocities and wall heat fluxes ranging from 42% to 45% of CHF. Experiments are performed in which constant and equal heat fluxes are supplied from the heating walls, with local wall temperatures measured at several axial locations and used to determine axial variations of local heat transfer coefficient. Additionally, detailed information on flow pattern and phase distribution is captured by high-speed video through the channel’s transparent sidewalls. Experimental values for key fluid flow and thermal parameters are compared to predictions of 2D computational simulations using ANSYS FLUENT. The computations are based on the multi-phase volume of fluid (VOF) model combined with an appropriate phase change model, and account for conjugate heat transfer along the heating walls. Computed transverse profiles and stream-wise variations of void fraction, flow velocity, and wall temperature are examined. The computed results show reasonable agreement with experimentally captured interfacial behavior as well as heat transfer parameters. Computed cross-sectional temperature profiles for the mixture show non-equilibrium subcooled boiling with steep temperature gradients near the heating walls and low temperature liquid in the core region. This study is concluded by exploring limitations of the 2D model used.

Simultaneous prediction of dryout heat flux and local temperature for thin film evaporation in micropillar wicks

Porous wicks are of great interest in thermal management because they are capable of passively supplying liquid for thin film evaporation, a promising method to reliably dissipate heat in high performance electronics. While dryout heat flux has been well-characterized for many wick configurations, key design information is missing as many previous models cannot determine the distribution of evaporator surface temperature. Temperature gradients are inherent to the passive capillary pumping mechanism since the shape of the liquid/vapor interface is a function of the local liquid pressure, causing spatial variation of permeability and heat transfer coefficient (HTC). Here, we present a comprehensive modeling framework for thin film evaporation in micropillar wicks that can predict dryout heat flux and local temperature simultaneously. Our numerical approach captures the effect of varying interfacial curvature across the micropillar evaporator to determine the spatial distributions of temperature and heat flux. Heat transfer and capillary flow in the wick are coupled in a computationally efficient manner via incorporation of parametric studies to relate geometry and interface shape to local permeability and HTC. This model predicts notable variations of HTC (∼30%) across the micropillar wick, highlighting the significant effects of interfacial curvature. Further, we are able to quantify the tradeoff associated with enhancing either dryout heat flux or HTC by optimizing geometry. Our model provides all of the information needed to guide the design and optimization of micropillar wicks by resolving evaporator temperature distributions in addition to dryout heat flux.

Numerical study on heat transfer performance of micro-channel gas coolers for automobile CO2 heat pump systems

This study aims to investigate the supercritical heat transfer characteristics of compact micro-channel gas coolers applied in automobile CO2 heat pump systems. A simulation model of automobile gas cooler was developed by using segment-by-segment method, and validated by experimental results. Using this model, under the typical heat pump operation conditions, the temperature distributions along the CO2 refrigerant flow path were predicted, and the variation of heat transfer coefficient was analyzed. Furthermore, the impacts of operation conditions such as refrigerant inlet pressure and temperature, as well as configuration parameters such as depth and pass number on the heating performance were discussed. Results show that the established model has a good prediction accuracy for heating capacity of gas cooler. It is found that there is unfavorable air temperature inhomogeneity on the airside outlet surface of gas cooler.

Development of a Space Heating Model Suitable for the Automated Model Generation of Existing Multifamily Buildings—A Case Study in Nordic Climate

Building energy performance modeling is essential for energy planning, management, and efficiency. This paper presents a space heating model suitable for auto-generating baseline models of existing multifamily buildings. Required data and parameter input are kept within such a level of detail that baseline models can be auto-generated from, and calibrated by, publicly accessible data sources. The proposed modeling framework consists of a thermal network, a typical hydronic radiator heating system, a simulation procedure, and data handling procedures. The thermal network is a lumped and simplified version of the ISO 52016-1:2017 standard. The data handling consists of procedures to acquire and make use of satellite-based solar radiation data, meteorological reanalysis data (air temperature, ground temperature, wind, albedo, and thermal radiation), and pre-processing procedures of boundary conditions to account for impact from shading objects, window blinds, wind- and stack-driven air leakage, and variable exterior surface heat transfer coefficients. The proposed model was compared with simulations conducted with the detailed building energy simulation software IDA ICE. The results show that the proposed model is able to accurately reproduce hourly energy use for space heating, indoor temperature, and operative temperature patterns obtained from the IDA ICE simulations. Thus, the proposed model can be expected to be able to model space heating, provided by hydronic heating systems, of existing buildings to a similar degree of confidence as established simulation software. Compared to IDA ICE, the developed model required one-thousandth of computation time for a full-year simulation of building model consisting of a single thermal zone. The fast computation time enables the use of the developed model for computation time sensitive applications, such as Monte-Carlo-based calibration methods.