Investigation Of Heat Transfer Mechanism Research Articles

Modeling of Solidification Process and Investigation of Heat Transfer Mechanisms in Micron-Sized Spherical Cu Particles

Modeling of Solidification Process and Investigation of Heat Transfer Mechanisms in Micron-Sized Spherical Cu Particles

Numerical investigation of heat transfer mechanism in an embedded PCM-TEG for better overall performance under the fluctuating heat source

Using phase change materials (PCM) can effectively ensure the stable operation of semiconductor thermoelectric generators (TEGs). Conventional PCM-TEG structures exhibit relatively poor output performance due to energy barriers caused by low thermal conductivity. The embedded PCM-TEGs proposed in this study can address this issue well by adopting variable structure composite forms of PCM and TEG, resulting in a more complex heat transfer mode than traditional systems. To evaluate embedded systems' characteristics and performance enhancement methods, this paper first establishes a multiphysics mathematical model for embedded PCM-TEGs. Then, employing the TOPSIS method, an optimal embedded structure that considers system output and stability is obtained. Finally, we investigate the effects of key parameters on system performance, including PCM volume, latent heat, melting point, and metal-added composite materials. The CCC PCM-TEG (embedded PCM-TEG with PCM closer to the cold side) exhibits superior performance with a 28.6 % increase compared to conventional structures. For a given PCM volume, embedded structures with equal heights show better performance due to weak influence from asymmetric configurations on internal heat flow direction. Unlike traditional PCM-TEGs, metal-added composite PCMs in embedded structures reduce the temperature difference range across TEGs and thus deteriorate system performance.

Numerical investigation of heat transfer mechanisms in microfibrillated cellulose cryogels: effects of concentration, porosity, and thickness

Numerical investigation of heat transfer mechanisms in microfibrillated cellulose cryogels: effects of concentration, porosity, and thickness

Application of Porous-Embedded shell and tube heat exchangers for the Waste heat Recovery Systems

• Investigation of heat transfer mechanisms inside porous-filled STHX designs. • Detailed comparison over the velocity, temperature, and pressure fields between the porous-filled and traditional STHX designs. • Investigation of the influence of porous-foam on the thermofluidic properties of traditional STHX designs. • A comprehensive sensitivity analysis of the operational condition for the porous-filled and the traditional designs. Among the techniques offered to improve the efficiency of Waste Heat Recovery Shell and Tube Heat Exchangers (WHR-STHX), the porous-filling is reported as an effective technique to improve the heat transfer rate. In this regard, the current study performs a numerical investigation to evaluate the heat transfer performance and pressure loss of several porous-filled STHXs. However, the novelty of this study stems from identifing the mechanisms that manipulated the flow structures and heat transfer to achieve a homogeneous thermal distribution, not from achieving an optimized porous-filling. Furthermore, the industrial feasibility of the porous-filled designs was evaluated by comparing their performance with the conventional type in a wide range of thermofluidic conditions. Based on the results, both porous-filled designs have substantially attenuated the interfacial thermal jumps observed in the conventional STHX; thus, a uniform thermal distribution was achieved. Furthermore, the heat transfer efficiency of the porous-filled cases was improved up to 60% compared with the conventional type; however, they imposed higher pressure drop values. Fortunately, since partially porous-filling provided a lower amount of pressure drop (almost half of full-foam), the noted design was found to be appropriate for low-scale applications in which pressure drop could not be tolerated.

Numerical and Analytical Investigation of Heat Transfer Mechanisms and Flow Phenomena in an Intermediate Pressure Steam Turbine Blading During Startup

Abstract As a result of an ever-increasing share of volatile renewable energies on the worldwide power generation, conventional thermal power plants face high technical challenges in terms of operational flexibility. Consequently, the number of startups and shutdowns grows, causing high thermal stresses in the thick-walled components and thus reduces lifetime and increases product costs. To fulfill the lifetime requirements, an accurate prediction and determination of the metal temperature distribution inside these components is crucial. Therefore, boundary conditions in terms of local fluid temperatures as well as heat transfer coefficients (HTCs) with sufficient accuracy are required. As modern numerical modeling approaches, like 3D-conjugate-heat-transfer (CHT), provide these thermal conditions with a huge calculation expense for multistage turbines, simplified methods are inevitable. Analytical heat transfer correlations are thus the state-of-the-art approach to capture the heat transport phenomena and to optimize and design high efficient startup curves for flexible power market. The objective of this paper is to understand the predominant basic heat transfer mechanisms such as conduction, convection, and radiation during a startup of an intermediate pressure (IP) steam turbine stage. Convective heat transport is described by means of heat transfer coefficients as a function of the most relevant dimensionless, aero-thermal operating parameters, considering predominant flow structures. Based on steady-state and transient CHT simulations, the heat transfer coefficients are derived during startup procedure and compared to analytical correlations from the literature, which allow the calculation of the heat exchange for a whole multistage in an economic and timesaving way. The simulations point out that the local convective heat transfer coefficient generally increases with increasing axial and circumferential Reynolds' number and is mostly influenced by vortex systems such as passage and horseshoe vortices. The heat transfer coefficients at vane, blade, hub, and labyrinth-sealing surfaces can be modeled with a high accuracy using a linear relation with respect to the total Reynolds' number. The comparison illustrates that the analytical correlations underestimate the convective heat transfer by approximately 40% on average. Results show that special correlation-based approaches from the literature are a particularly suitable and efficient procedure to predict the heat transfer within steam turbines in the thermal design process. Overall, the computational effort can be significantly reduced by applying analytical correlations while maintaining a satisfactory accuracy.

Parameter study at the PKL test facility on heat transfer mechanisms in the steam generator in presence of nitrogen, steam and liquid

PKL is a worldwide unique integral test facility for simulation of thermal-hydraulic phenomena, which can occur in Pressurized Water Reactor under abnormal or accident conditions. The test rig replicates the entire primary side and the relevant parts of the secondary side of western-type PWR plant in the scale of 1:1 in heights and 1:145 in volume and power. The analyses that have been carried out for the last 40 years have always reflected safety issues addressed in the debate of nuclear community. Among goals of experimental programs conducted at the facility there are demonstration of safety margins, investigation of thermal-hydraulic behavior of a plant under accident conditions, verification of accident management procedures as well as provision of data for thermal-hydraulic codes validation. This paper provides a detailed analysis of experiment dedicated to investigation of heat transfer mechanisms in steam generator in presence of non-condensable gas, steam and liquid. Such conditions can occur in a PWR in case of loss of residual heat removal system under cold shutdown conditions. Probabilistic safety assessments indicated that this accident contributes to much larger core damage frequency than originally anticipated. Therefore, an increasing attention has been paid to this accident scenario in the recent years.

Investigation of heat transfer mechanisms among particles in horizontal rotary retorts

The horizontal rotary retort is one of the important equipment in oil shale retorting by solid heat carrier technology, in which the heating of oil shale plays a crucial role in shale oil production. In this work, a comprehensive heat transfer model including both conduction and radiation under contact and non-contact conditions was proposed to numerically investigate the heat exchange among particles at five different pathways by employing the discrete element method (DEM). The effects of particle physical parameter (particle size) and operating conditions (rotational speed and fill ratio) on thermal characteristics such as oil shale heating time, thermal rate, thermal index and heat flow were deeply analyzed. In summary, the heat flow generated by particle-fluid-particle heat conduction contributed 66–71% of the total heat flow, the radiative flow produced by particle-particle heat radiation was 22–30%, and the thermal contribution of particle-particle heat conduction was the least, accounting for only 6–7%.

Ground-Based Near-Space-Oriented Spray Cooling: Temperature Uniformity Analysis and Performance Prediction

Thermal control systems in near-space-oriented vehicles are bound to be different from those on either traditional airplanes or typical space shuttles owing to the extremely thin atmosphere caused by the high altitude and awfully hot air produced by the aerodynamic heat. The tough thermal environment encountered by the vehicle will pose great challenges for the thermal control system deployed in the near-space platform, which is hazardous for the onboard electrodriven equipment. Therefore, it is imperative to invent a novel cooling scheme for the reliable operation of the device. For thermally protecting the onboard permanent magnet synchronous motor in a rudder cabin, a novel large-space spray cooling system was proposed in a previous study by Wang, J. X., Li, Y. Z., Yu, X. K., Li G. C., and Ji, X. Y. ("Investigation of Heat Transfer Mechanism of Low Environmental Pressure Large-Space Spray Cooling for Near-space Flight System," International Journal of Heat and Mass Transfer, Vol. 119, April 2018, pp. 496–507); based on which, a further study of the experimental data is conducted in this paper. Temperature uniformities within the small-scaled motor and among different motors are studied, and strategies for enhancing the temperature uniformity among motors are proposed. In addition, thermal predictions of the small-scaled permanent magnet synchronous motor for both of the flash-boiling subcooled regimes using the backpropagation neural network are developed. A maximum relative error of is observed, which illustrates the superiority of the current model as compared with the fitting correlation developed in the previous study.

Multi-objective optimization of heat transfer mechanisms of microcellular polymeric foams from thermal-insulation point of view

In this study, a comprehensive investigation of heat transfer mechanisms of polymeric foams is performed using response surface methodology (RSM). Foam density, cell size and strut fraction are selected as variable parameters. A regression model is obtained to predict the overall thermal conductivity using analysis of variance (ANOVA) tool of RSM. The results indicate that this regression model with R2 = 95.22% and having an error smaller than 4% has an appropriate performance to estimate the overall thermal conductivity. Foam density is the most effective parameter on the overall thermal conductivity with contribution of 69.8%, as ANOVA results show. Reduction of foam density and cell size leads to decreasing the thermal conductivity while it is also decreased by increasing the strut fraction. It is reported for the first time that the cell walls are dominant components in comparison to struts in heat transfer mechanism by solid phase. On the other hand, it is discovered that the strut component is the predominant part in absorption phenomenon. The overall thermal conductivity is 23.69 mW/mK at the optimum condition (i.e. foam density of 25 kg·m−3, cell size of 1 μm and strut fraction of 0.2 in the studied range) using multi-objective optimization tool of RSM.

Investigation of heat transfer mechanism of low environmental pressure large-space spray cooling for near-space flight systems

There is an imperative need to employ the on-board electro-driven equipment since the concept of all-electric and more-electric technology has been applied in the design of near-space flight systems. This application will inevitably bring a dramatic increase in the waste heat because of incomplete energy conversion, which will easily cause an overheating in the electrical device. What’s worse, the near-space flight system will encounter extremely tough thermal environment where the atmosphere is rarefied due to the high altitude and the temperature of the introduced air into the plane is awfully high owing to the aerodynamic heat, both of which will pose a great challenge for the near-space thermal control system. Aiming to design an efficient thermal protection strategy of the on-board permanent magnet synchronous motor for the steering of major flight control which operates constantly in the near-space cruise stage, this paper proposes a novel low environmental pressure large-space spray cooling system. Revolving the scheme, a small-scaled experimental prototype was designed and established, upon which thermal tests were conducted to estimate influence of several parameters such as environmental pressure, nozzle inlet temperature and spray volumetric flow rate upon the cooling performance. Both flash boiling and subcooled spray phenomena were discussed, results of which suggests the former is more desirable due to the temperature uniformity and high coolant utilization. In addition, capable of providing performance predictions and guidances for future full-scaled tests, an empirical experimental correlation with the relative error of only ±8% was acquired on the basis of the data in the flash boiling region.

Rainbow schlieren-based investigation of heat transfer mechanisms during isolated nucleate pool boiling phenomenon: Effect of superheat levels

Experimental investigation of various heat transfer mechanisms associated with isolated nucleate pool boiling have been presented. Measurements have been made in a complete non-intrusive manner using rainbow schlieren deflectometry technique. Boiling experiments have been performed for two levels of superheat with the bulk fluid maintained under saturated conditions. The rainbow schlieren images have first been subjected to qualitative interpretation wherein various sub-processes associated with the boiling phenomenon, such as development of thermal boundary layer on the substrate surface, inception of single bubble, growth of the vapor bubble till it departs, and scavenging of the superheat layer following the bubble departure have been discussed. Contributions of individual sub-processes towards the overall heat transfer rates achieved for a given superheat level have been determined through quantitative analysis of the images. Schlieren observations revealed the effect of varying superheat levels on parameters such as bubble diameter and departure time. Detailed heat transfer analysis revealed the dominance of evaporative heating in contributing towards the overall heat transfer rates. On the other hand, the contribution of natural convection from the heated substrate was found to be relatively small. In quantitative terms, the evaporative heating was seen to have an individual contribution as high as ≈66% to the overall heat transfer and ≈88% to the growth of the vapor bubble in the case of superheat level of 7 °C.

Dissipative particle dynamics investigation of heat transfer mechanisms in Al2O3-water nanofluid

The current paper applied dissipative particle dynamics (DPD) approach to investigate the effect of nanoparticles on the heat transfer mechanisms in natural convection using Al2O3-water nanofluids. The study investigated in detail the effect of nanoparticle concentration on the random heat flux (i.e., Brownian motion) and the total heat flux. The DPD model considered the viscosity and the thermal conductivity of the Al2O3-water nanofluid to be dual function of temperature and volume fraction of nanoparticles. The study covered a wide range of nanoparticles (1% ≤ φ ≤ 7%) and three Rayleigh numbers were considered, which are Ra = 104, Ra = 5 × 104, and Ra = 105. The DPD results predicted an enhancement in heat transfer due to the addition of nanoparticles. However, the results revealed that the relative of enhancement of the total heat flux in the cavity is more effective at low Rayleigh numbers than at high Rayleigh numbers. The regions around the hot wall of the cavity are found to experience the maximum enhancement in heat transfer in the cavity whereas the region adjacent to the cold wall experienced a deterioration in heat transfer due to the addition of nanoparticles. Also, the DPD results revealed that the role of Brownian motion in the vicinity of hot and cold walls was negligible where the ratio of random heat flux to the total heat flux was below 5%. However, this ratio reached a very large value around the center of the cavity and the bottom wall of the cavity, where this value at low Rayleigh number and high volume fraction of nanoparticles exceeded 65%. Moreover, the study revealed that the random heat flux enhances with increase of volume fraction of nanoparticles.

Fundamental investigation of heat transfer mechanisms between a rolling sphere and a plate in OpenFOAM® for laminar flow regime

The heat transfer between particles and walls has been topic of research for several decades. In most cases existing correlations were developed empirically for special conditions and applications like fluidized beds or rotary kilns. With a deeper fundamental understanding of the occurring heat transfer mechanisms both design of existing and development of innovative apparatuses, could be more straightforward since the mentioned existing correlations cannot be applied flawlessly to other conditions, due to specific simplifications and assumptions. The aim of this work is to investigate the fundamental heat transfer mechanisms between spherical particles rolling on a plate on the microscale with the lowest level of simplifications. For the identification of the dominating heat transfer mechanisms an approach with Computational Fluid Dynamics (CFD) simulations is chosen. The fundamental investigations are carried out using a basic geometry of a moving sphere on a plate. Due to the lack of a solver which is able to describe the system of a moving sphere on a plate, a new solver is developed in OpenFOAM®. The solver is validated against an existing model as well as own experimental results. Simulations in the laminar flow region show good agreement. Analogically to literature the main transport resistance is identified in the gap between plate and sphere. The heat conduction through the gas layer between sphere and plate is identified as the dominating mechanism.

Improvement of surface grinding process performance of CK45 soft steel by minimum quantity lubrication (MQL) technique using compressed cold air jet from vortex tube

Consumption of cutting fluids imposes high costs on industries. Furthermore, they contaminate the environment and are harmful to human health. Minimum quantity lubrication technique (MQL) is an attractive alternative approach to reduce cutting fluids consumption, improve efficiency of cutting fluids at machining zone and use of harmless fluids. However, this technique faces cooling limitation in grinding. The first purpose of this study is a comprehensive investigation of heat transfer mechanism in MQL technique by its temperature numerical simulation. The second purpose is improvement of cooling ability of MQL air jet by using a simple and inexpensive vortex tube. To do this end, a system was designed and manufactured to measure the convection heat transfer coefficient of different conditions of cold air with MQL (CAMQL). The results of convection heat transfer tests show 95% share of compressed air in heat transfer. Also, the results indicate that air pressure is a more important factor than temperature in cooling process at high thermal power. The results of temperature numerical simulation show that by increasing pressure, the increasing rate of convection heat transfer coefficient decreases. Also, the temperature cooling ability of colder air with lower pressure is perceptible at low thermal power. In surface grinding of CK45 soft steel, CAMQL in comparison with dry and conventional fluid cooling leads to significant reduction of tangential grinding force and friction coefficient. But in general, except in the case of optimum condition which has the highest heat transfer coefficient, surface finish is worse. This is attributed to low heat transfer coefficient of gases at lower pressures.

Numerical investigation of heat transfer mechanism in a vertical shell and tube latent heat energy storage system

The thermal behavior and heat transfer characteristics of a vertical cylindrical shell and tube latent heat thermal energy storage (LHTES) unit are investigated using a pure thermal conduction model and a combined conduction-convection heat transfer model, respectively. The results from numerical simulation are validated with published experimental data which indicate that the combined convection and conduction model can better describe the energy transfer in the phase change materials (PCMs) during melting process. In contrast, heat transfer by conduction is more significant during the solidification process. In terms of total solidification time, the two models show little difference.

A Numerical Investigation of Heat Transfer Mechanisms in Gas-Solid Fluidized Beds Using the Two-Fluid Granular Temperature Model

In this work, the two-fluid granular temperature model is used to investigate the heat exchanged between a heated wall and a gas-solid fluidized bed. Numerical simulations were performed in 2-D and 3-D fluidized beds using a solid phase effective thermal conductivity correlation based on the granular temperature. The heat exchange in the bubbles' wake is investigated by tracking the train of bubbles that rises along the heated wall. Large heat transfer coefficients were obtained in the rear wake region of bubbles due to relatively larger granular temperature there and intense particle circulation.