Convective Heat Transfer Mechanism Research Articles

Unified model of heat transfer in the multiphase flow in Steam Assisted Gravity Drainage process

Steam assisted gravity drainage (SAGD) is an effective method for bitumen and heavy oil reservoirs. In SAGD process, steam is injected into the reservoir with a high pressure from the upper horizontal well, and the injected steam will heat the surrounding oil sands through a conduction and convection heat transfer process. In this paper, a novel unified model is developed to investigate the heat transfer mechanism in SAGD process. Typically, the multiphase flow assumption at the edge of steam chamber is introduced in our model. And the effect of both the conduction and convection heat transfer in steam chamber are considered. Especially, in this model, the convective heat transfer process caused by pressure difference and gravity is studied. Then the simulation results are compared against the UTF field data to confirm the accuracy of our model. Results indicate that our model can match the UTF field data very well, and it can be used to investigate the heat transfer process in SAGD process. It is found that the oil production rate decreases as the interface velocity increases. The optimal interface velocity is about 1.5–2.5 cm/d. In this range, the oil production rate is about 0.4 m3/(m·d) and oil cut is approximately 31%. The variations of condensate liquid mobility and density lead to that oil production will decrease as the pressure difference increases. Finally, it can be concluded that a suitable injection pressure should be chosen to guarantee the optimal interface velocity in SAGD process. This new model will benefit to understanding the convective heat transfer mechanism and guiding the field production behavior in SAGD process.

A New Mathematical Model to Understand the Convective Heat Transfer Mechanism in Steam-Assisted Gravity Drainage Process

Steam-assisted gravity drainage (SAGD) process has been an optimized method to explore heavy oil reservoirs in the world. The oil viscosity reduction and gravity force near the interface of steam–chamber are the main development mechanisms. In classical models, conductive heat transfer plays the only or dominant role in the heat transmission from high-temperature steam to low-temperature oil sands. Although some mathematical studies have paid attention to the convective heat transfer, the role of heat transfer by flowable oil normal to the steam–chamber interface has been given little attention. In SAGD, the viscosity of bitumen can be reduced by several orders of magnitude by the release of latent heat from injected steam. In this study, an analytical model is developed for the heat transfer process induced by flowable oil. Also, in order to accurately simulate the oil viscosity characteristics in steam–chamber, a correlation between oil viscosity and pressure is proposed. Results indicate that the oil mobility plays an important role on the flow normal to interface when the distance is smaller than 6 m. Even under the most extreme circumstances (μw = 0.1127 cp), the flowing of oil normal to steam–chamber interface also cannot be ignored. Comparing to Irani and Ghannadi model, it can be easy to draw the conclusion that the new model consists with the underground test facility (UTF) field data much better. This new analytical model will benefit to understanding the convective heat transfer mechanism in SAGD process.

Numerical Study of Convective Heat Transfer in the End Regions of a Totally Enclosed Permanent Magnet Synchronous Machine

This paper proposes a numerical approach for the accurate estimation of convective heat transfer coefficients in the end region of an integrated starter generator (ISG) used in a mild-hybrid vehicle. A simplified numerical model based on CFD methods is developed to provide a qualitative understanding of the mechanism of convective heat transfer over a large operation range. The effect of the rotation speed on the variation of the convective heat transfer coefficients inside the end region is thoroughly investigated. Heat flux paths are also identified for the entire operation range. CFD calculation results are used to provide an enhancement to Schubert's correlation for the different surfaces delimiting the end region of the ISG. Subsequently, general correlations are defined using averaged Nusselt numbers as a function of Reynolds numbers. Then, a detailed lumped thermal network is developed which estimates correctly the temperature distribution in different areas of the electrical machine. The obtained numerical data are validated using local temperature measurements.

Switchable heat transfer mechanisms of nucleation and convection by wettability match of evaporator and condenser for heat pipes: Nano-structured surface effect

A novel strategy was proposed for the improvement of vapor chamber heat pipe (HP) performance. The mastoid process array, wettability control of heat pipe and control of liquid charged into heat pipe are comprehensively used to manage the phase distribution. Thus, the two dominant heat transfer mechanisms of nucleation and convection can be switched. The objective of this strategy is to maintain nucleation mechanism but suppress convection mechanism. Holding the nucleation mechanism, our newly developed heat pipe behaves excellent thermal sensitivity with respect to heating loads. Our idea was thoroughly verified by experiments, in which three HP samples were fabricated and tested: #1 equal wettability sample, #2 moderate wettability difference sample and #3 opposite wettability sample. The wettability difference was generated with nano-structured surface. The #1 sample shows convection mechanism with heat transfer coefficients (HTCs) not changed versus heat fluxes, due to evaporator porous wick occupied by larger vapor content and liquid filmwise condensation on condenser surface. The super-hydrophilic evaporator and super-hydrophobic condenser (#3 sample) shows nucleation mechanism with HTCs significantly increased with increases in heat fluxes, due to large driving force to flood evaporator porous wick by water and expose condenser by vapor. Surprisingly, opposite wettability match HP had overall thermal resistances of only 1/3 to those of equal wettability HP. The comprehensive use of mastoid process array and controls of wettability and water amount charged into HP lowers heater temperatures by 30–40°C at high heat fluxes such as ~100W/cm2 on a large heater area of 1.4cm2, paving a new road for heat pipe performance improvement.

Multi-layer PCM solidification in a finned triplex tube considering natural convection

Efficient latent heat storage systems represent an important opportunity to improve the thermal performance and operational capability of industrial systems (e.g. solar thermal). In this paper, numerical study is performed to evaluate the heat transfer and solidification features, phase change period and temperature distribution of double-layer Phase Changing Materials (PCM) in a finned triplex tube. A two-dimensional finite volume numerical technique is used to solve the governing equations considering conduction and convection heat transfer mechanisms at a fixed Rayleigh number of 106. After comparing the results for single and double layer configurations, possible arrangements of two different PCMs are discussed in terms of different thicknesses of each PCM, fin sizes and different heat transfer fluid temperatures. Results are illustrated as the form of temperature, liquid fraction, stream function and velocity magnitude contours and their changes over the freezing time. Variation of liquid fraction values, average and minimum temperatures of layers in a double-layer PCM are reported for better insight into the heat transfer features of the latent heat thermal energy storage system to enable uniform discharging designs and balance the phase changing rate within the whole annulus.

Simulation of jet-flow solid fraction during spray forming

A numerical model was developed to simulate the jet-flow solid fraction of W18Cr4V high-speed steel during spray forming. The whole model comprises two submodels: one is an individual droplet model, which describes the motion and thermal behaviors of individual droplets on the basis of Newton’s laws of motion and the convection heat transfer mechanism; the other is a droplet distribution model, which is used to calculate the droplet size distribution. After being verified, the model was used to analyze the effects of parameters, including the initial gas velocity, deposition distance, superheat degree, and the ratio of gas-to-metal mass flow rates, on the jet-flow solid fraction. Finally, an equation to predict the jet-flow solid fraction directly and conveniently according to the parameters was presented. The values predicted by the equation show good agreement with those calculated by the numerical model.

Thermal-hydraulic characteristics and exergy performance in tube-on-sheet flat plate solar collectors: Effects of nanofluids and mixed convection

Flat plate solar collector (FPSC) is one of the most popular equipment among solar energy systems which can be utilized for heating of domestic or public buildings where the demands for hot water are quite indispensable. Although they have a number of benefits such as no need for sun tracking and low maintenance cost, their low thermal performance is considered as a hindrance in their extensive development. In this paper, a three-dimensional inclined tube-on-sheet flat plate solar collector is numerically modeled. The FPSC system is deemed to work under conjugated laminar mixed convection heat transfer mechanism while the operational fluid is selected to be Al2O3/water nanofluid. The influence of several parameters on the performance of the present FPSC, namely inlet alumina/water nanofluid temperature, and volume concentration, heat flux absorbed by the absorber plate, dimensionless numbers consist of Reynolds (Re), Grashof (Gr), Prandtl (Pr), and Richardson (Ri) are discussed on the heat transfer, fluid flow, and entropy generation. Furthermore, the effects of friction factor inside the riser as well as the pressure drop are taken into the account in this research. Results revealed that increasing the volume fraction of nanofluid at a fixed Reynolds number declines the outlet temperature whereas lowering the Reynolds number at a same volume fraction gives rise to higher outlet temperature. From heat transfer standpoint, it is observed that in a constant Reynolds number by increasing nanoparticle volume fraction, the Nusselt number falls while the heat transfer coefficient rises. In addition, rising the Reynolds number eventuates in friction factor reduction while the pressure drop augments. Moreover, from the obtained outcomes, it is figured out that generally using alumina/water nanofluid provides lower total entropy generation, in comparison with the water. Simulation data reveal that alteration in the heat flux does not have any sensible effects on the friction factor.

A Conjugate Natural Convection Model for Large Scale Seasonal Thermal Energy Storage Units: Application in Mine Ventilation

Ventilation plays an important role in energy demands of mining industry. Using the immense volume of waste rock as thermal energy storage mass to shave seasonal air temperature oscillations is a genuine and practical solution to save on ventilation energy demands. The extensive size and considerable temperature gradients exhibited in such systems point to the importance of buoyancy-driven natural convection heat transfer mechanism. The present study investigates the effect of natural convection and identifies the order of significance of various heat transfer mechanisms. A Conjugate heat transfer and fluid flow model is proposed which includes both the porous rock mass placed in the rock-pit and the air positioned above it. The results of the model are compared and validated against the existing Non-Conjugate models. Using this novel model, significance of forced convection, driven by ventilation fan, and natural convection, driven by buoyancy, are compared. The model is also used to study how design parameters, such as position of intake air trenches and flow rate of fresh air, will affect energy savings.

Effect of tube diameter on boiling heat transfer and flow characteristic of refrigerant R32 in horizontal small-diameter tubes

This study investigated the effect of tube diameter on flow boiling characteristics of refrigerant R32 in horizontal small-diameter tubes with 1.0, 2.2, and 3.5 mm inner diameters. The boiling heat transfer coefficient and pressure drop were measured at 15 °C saturation temperature. The effects of mass velocity, heat flux, quality, and tube diameter were clarified. The flow pattern of R32 for adiabatic two-phase flow in a horizontal glass tube with an inner diameter of 3.5 mm at saturation temperature of 15 °C was investigated. Flow patterns such as plug, wavy, churn, and annular flows were observed. The heat transfer mechanisms of forced convection and nucleate boiling were similar to those in conventional-diameter tubes. In addition, evaporation heat transfer through a thin liquid film in the plug flow region for low quality, mass velocity, and heat flux was observed. The heat transfer coefficient increased with decreasing tube diameter under the same experimental condition. The fictional pressure drop increased with increasing mass velocity and quality and decreasing tube diameter. The experimental values of the heat transfer coefficient and frictional pressure drop were compared with the values calculated by the empirical correlations in the open literature.

Effect of Reverting Channels on Heat Transfer Performance of Microchannels with Different Geometries

This study investigated the effect of reverting microchannels inside a heat sink on increase of cooling rate as well as the effect of their different configurations on maximum temperature and pressure drop. Based on the convection heat transfer mechanism, selection of the embedded microchannels’ configurations including circular, square and triangular ones, was studied for geometric optimization of the discussed heat sinks. The goal was to minimize the thermal resistance through optimizing the geometries. The volume ratio was defined as the ratio of the volume taken up by microchannels to the solid volume (portion of the heat sink not occupied by microchannels) and was considered as 0.05. According to the results, in addition to obtaining a more uniform temperature distribution, reverting channels remarkably reduced the maximum temperature. Moreover, the heat sink with square shape showed less thermal resistance as compared to the other two geometries.

Experimental investigation on the convective heat transfer enhancement in tubes with cross-helix profile wall corrugation

Wall corrugation is a popular heat transfer enhancement technique since it acts as a disturbance source in the flow that significantly enhances the thermal performance of the tube section with a limited pressure drop augmentation if compared to other passive techniques, such as the ones based on insert devices. In the hereby presented study nine pipes characterised by a cross-helix type corrugation were tested: this kind of corrugation, that was obtained by rolling twice the same tube with two helical corrugations evolving along opposite directions, showed a performance that exceeded the single helix-type corrugation behaviour and moreover presented an earlier transition to unstable regime. In the analysis the effect of the cross-helix type corrugation profile on the forced convection heat transfer mechanism was experimentally investigated in the Reynolds and Prandtl number range 25÷1000 and 115÷150 respectively. In particular, the effect of both the corrugation depth and the corrugation pitch were analysed. The results were compared with other types of wall corrugation and with the predictions for the smooth tube in order to point out the achieved heat transfer enhancement.

Field Synergy Analysis of Thermal Storage Effect of Solar Energy Storage Tank

The internal structure of solar energy storage water tank partition design was carried out in this paper. The energy storage tank with different internal structure had been simulated to analysis convective heat transfer mechanism in the water tank by using CFD method. The temperature stratification mechanism of water tank had been deeply analysed to improve the availability of storage tank as the goal based on the heat transfer and flow field synergy theory. The calculation results showed that the partition plate in the water tank could obviously improve the effect of hot and cold stratification of the water tank, so that a good effect of cold and hot stratification from left to right could be achieved; The influence of flow field on temperature field had been studied by analyzing the degree of synergy between velocity vector and temperature gradient vector, The flow resistance, drag reduction mechanism and influencing factors in the water tank were studied by analyzing the synergy between the velocity vector and the pressure gradient vector; Through the adjustment of internal structure and the change of water flow and heat transfer and pressure gradient direction, that was to change the angle between the velocity and the temperature gradient and the velocity and pressure gradient, and It could effectively improve the energy storage and delamination effect of the water tank; and provided a theoretical basis for the future design of efficient solar energy storage tank with low flow resistance.

Experimental process investigation of a latent heat energy storage system with a staggered heat exchanger with different phase change materials for solar thermal energy storage applications

This work investigates melting and solidification processes of four different Phase Change Materials (PCM) used as latent heat thermal storage system. The experimental rig was consisted of an insulated tank, filled with the under investigation PCM, a staggered heat exchanger to supply or extract heat from the PCM cavity and a water pump to circulate Heat Transfer Fluid (HTF). Both charging (melting) and discharging (solidification) processes were conducted for two different HTF flow rates. The main scope of this work was to develop a first approach and to investigate the behaviour of PCM under various load conditions (different HTF flow rates). Results show that different HTF flow rates affect melting and solidification time periods; in both processes time was reduced while HTF flow rate was increased but in differentways due to the transition from conduction to convection heat transfer mechanisms.

Vortex dynamics and mechanisms of heat transfer enhancement in synthetic jet impingement

When confined flow is oscillated from and into a quiescent volume, a periodic coherent flow that resembles that of a conventional jet can be generated. Such a jet, termed a synthetic jet, has been investigated for thermal management by causing it to impinge onto a heated surface. Because of its fluctuating nature, the impinging jet thus formed is dominated by vortices that are advected towards the surface. This surface-vortex interaction is key to understanding the fundamental mechanisms of convective heat transfer by the impinging synthetic jet, which motivates this work along with the search for the improvement of the system thermal efficiency. A canonical geometry was developed to investigate the flow and heat transfer of a purely oscillatory jet that is not influenced by the manner by which it is produced. The unsteady two-dimensional Navier-Stokes equations and the convection-diffusion equation were solved using a finite volume approach in order to capture the complex time dependent flow field. The Q-criterion (Hunt et al., 1988), which defines vortices as connected fluid regions with positive second invariant of the velocity gradient tensor was utilized to identify vortices without ambiguity. A definition of the jet characteristic velocity was developed rigorously based on the vortex dynamics produced by the jet. It is equivalent to the common definition accepted in the literature, which has been successfully used to match the dynamics of synthetic and steady jets, but which was developed using heuristic reasoning. When the primary vortex advects in a direction parallel to the target surface it gives rise to a secondary vortex with opposite net vorticity. This secondary vortex is largely responsible for enhancement of the heat transfer within the wall jet region. Under certain conditions, vortex coalescence occurs, leading to degradation in the heat transfer enhancement due to the reduction in the number of secondary vortices interacting with the heated surface. By understanding, quantifying and predicting the mechanisms that drive the phenomenon of vortex merging, optimum conditions of operation are demonstrated, ultimately leading to higher efficiencies by maximizing the heat transfer at similar pumping costs.

Nonlinear observer-based Lyapunov boundary control of distributed heat transfer mechanisms for membrane distillation plant

This paper presents a nonlinear observer-based Lyapunov control for a membrane distillation (MD) process. The control considers the inlet temperatures of the feed and the permeate solutions as inputs, transforming it to boundary control process, and seeks to maintain the temperature difference along the membrane boundaries around a sufficient level to promote water production. MD process is modeled with advection diffusion equation model in two dimensions, where the diffusion and convection heat transfer mechanisms are best described. Model analysis, effective order reduction and parameters physical interpretation, are provided. Moreover, a nonlinear observer has been designed to provide the control with estimates of the temperature evolution at each time instant. In addition, physical constraints are imposed on the control to have an acceptable range of feasible inputs, and consequently, better energy consumption. Numerical simulations for the complete process with real membrane parameter values are provided, in addition to detailed explanations for the role of the controller and the observer.

Assessing thermal conductivity of composting reactor with attention on varying thermal resistance between compost and the inner surface

Dynamic estimation of heat transfer through composting reactor wall was crucial for insulating design and maintaining a sanitary temperature. A model, incorporating conductive, convective and radiative heat transfer mechanisms, was developed in this paper to provide thermal resistance calculations for composting reactor wall. The mechanism of thermal transfer from compost to inner surface of structural layer, as a first step of heat loss, was important for improving insulation performance, which was divided into conduction and convection and discussed specifically in this study. It was found decreasing conductive resistance was responsible for the drop of insulation between compost and reactor wall. Increasing compost porosity or manufacturing a curved surface, decreasing the contact area of compost and the reactor wall, might improve the insulation performance. Upon modeling of heat transfers from compost to ambient environment, the study yielded a condensed and simplified model that could be used to conduct thermal resistance analysis for composting reactor. With theoretical derivations and a case application, the model was applicable for both dynamic estimation and typical composting scenario.

Temperature Calculation for Laser Sintering of Titanium and Niobium Taking into Account Properties Change due to Powder Layer Shrinkage

The model of laser sintering with regard to explicit porosity evolution of powder layer has been suggested. Due to porosity reduction the shrinkage occurs, thermal - physical properties (the heat capacity, thermal conductivity coefficient) and reflection coefficient of laser irradiation change. It has been shown that additional mechanism of convective heat transfer connects with powder layer thickness evolution. It was detected that model obeys some prognostic properties and allows to reveal the area of technological parameters where homogeneous properties of sintered layer form.

Numerical analysis of the laminar forced convective heat transfer in coiled tubes with periodic ring-type corrugation

Wall curvature and wall corrugation represent two of the most used passive techniques to enhance convective heat transfer. The effectiveness of wall curvature is due to the fact that it gives origin to a secondary fluid motion orthogonal to the main flow, while wall corrugation is used to disrupt the development of the boundary layers, by enhancing the convective heat transfer mechanism. The compound use of the two techniques has been investigated in literature, mainly experimentally, but further investigation is still needed. In particular, it has been experimentally observed that this compound enhancement technique brings an additional heat transfer augmentation in the majority of applications whereas in the very low Reynolds number range the surface average performances of corrugated coils are lower than the one shown by smooth wall coils.This paper deepened the knowledge on this phenomenon presenting a numerical investigation of the effect induced by a periodic ring-type corrugation on the laminar convective heat transfer in coiled tubes. The study considered the laminar flow in the Reynolds and Dean number range 25-100 and 6-24 respectively. The investigation was particularly focused on the Dean's vortices destruction mechanism, induced by the wall corrugation and on the consequent breakdown of the average Nusselt number.

Numerical investigation on using of nanofluid in a water-cooled photovoltaic thermal system

Photovoltaic thermal collector or PV/T is a new hybrid technology that produces heat and electricity simultaneously. The objective of the current study is to evaluate the effects of utilizing nanofluid on the performance of PV/T system. The considered coolant fluids in the simulation are pure water, Ag-water nanofluid and Alumina-water nanofluid. The proposed model includes a water riser tube and an absorber plate to consider the conduction and convection heat transfer mechanisms. The numerical results were obtained using the computational fluid dynamics (CFD) by employing conjugate heat transfer. The effects of the nanoparticle volumetric concentration and inlet fluid velocity on the efficiency and heat transfer enhancement were simulated. In this paper, the Second Law analysis of a water-cooled PVT is also represented, based on results of simulations. The results show that the efficiencies (the first and second laws of thermodynamics) and heat transfer coefficient increase by increasing the nanoparticle volume fraction. The maximum increase percentage of heat transfer coefficient versus volume fraction for alumina-water and Ag-water nanofluid are 12% and 43%, respectively. Increasing the inlet fluid velocity improves the heat transfer performance of PV/T system. Increasing the heat transfer coefficient with respect to the inlet fluid velocity for alumina-water nanofluid at φ=5% was in the range of 8–10%, in comparison to pure water. While this value for Ag-water nanofluid at the same volumetric fraction is significantly greater than alumina-water, which varies from 28 to 45%. For the purpose of validating the results of the presented method, a comparison study was carried out with the experimental results in the literature. The comparison shows an appropriate agreement with the results of the literature.

Natural convection heat transfer of molten salts around a vertically aligned horizontal cylinder set

The heat transfer from the vertical arrays of a set of equally spaced cylinders in molten salts is studied numerically to obtain the laminar natural convection heat transfer mechanism of molten salts around a vertically aligned horizontal cylinder set. Simulations are performed for arrays of 2–10 horizontal cylinders at a Rayleigh number based on a cylinder diameter between 2×103 and 5×105. Results show that the natural convective heat transfer of molten salts from the bottom cylinder of the array remains the same as that from a single cylinder. By contrast, the downstream cylinders may either be enhanced or reduced mainly depending on their location in the array and on the tube spacing. Heat transfer dimensionless correlating equations are proposed for any individual cylinder in the two vertically aligned horizontal cylinders. The heat transfer mechanism from the horizontal cylinders set in a vertical array is also simulated, and the results show that cylinder spacing can influence the average heat transfer rate around the whole tube array. Thus, in real applications, adjusting the cylinder spacing better enhances the average heat transfer from the whole tube array.