Transient Heat Transfer Phenomena Research Articles

Thermal and structural improvements of a motorcycle piston using a fully coupled thermo-mechanical FEM simulation

This paper aims to conduct thermo-mechanical simulations using the finite element method (FEM) to enhance the design of the piston in a single-cylinder, water-cooled motorcycle engine. The finite element model of the piston was developed to represent its thermo-mechanical behavior under combustion processes and transient heat transfer phenomena. Realistic engine boundary conditions were applied using the non-linear dynamic tool in Solidworks design software. Two different design configurations were investigated concerning the oil jets and the upper face of the piston pins. The results revealed lower piston temperatures by 12.2% as three oil jets targeted the upper region of the piston bosses. Additionally, an optimum radius fillet in the upper face of the piston holes showed potential for minimizing thermo-mechanical stresses. This finding suggests that optimizing the piston design can reduce developed thermo-mechanical stresses while addressing elevated temperatures.

A system-level multi-field coupling algorithm for regenerative cooling thrust chamber of a LOX/methane rocket engine

A systematic and hierarchical multi-physics coupling simulation method is proposed to model the combustion, combustion-gas transonic flow, coolant transcritical flow, and transient heat transfer phenomena inside the regenerative cooling thrust chamber subsystem of a LOX/methane rocket engine. By combining this algorithm with a self-developed platform, two distributed parameter modules are developed. Based on the ground test conditions of a LOX/methane engine, a verification case is established, and transient simulation studies are conducted. Comparisons with steady-state test data show that the calculated errors for coolant temperature rise and pressure drop are not higher than 4%. The simulation results reveal the locations where the transition from liquid to supercritical state occurs within the cooling jacket and provide three peril points of wall temperature on the thrust chamber. This study comprehensively reveals the occurrence of transcritical transient processes inside the cooling channels during the regenerative cooling process, laying the foundation for future entire system simulation.

Energy effectiveness analysis of desiccant coated heat exchanger based on numerical method

The conventional air-to-air heat pumps rely heavily on the condensation dehumidification principle to regulate air humidity, thereby severely limiting the system energy efficiency. Significant energy efficiency improvements can be realized by coating high-performing solid desiccants on both evaporator and condenser heat exchangers. Thus far, existing fundamental analysis developed to evaluate energy performance improvement and achieve accurate performance prediction of desiccant coated heat exchangers with two-phase refrigerant (DCHERs) has been impeded by the complex physical phenomena, approximations in the heat/mass transfer paths, and the severe reliance on the use of empirical correlations. Therefore, in this work, a mathematical approach has been judiciously developed based on mass, momentum, energy, and species conservation principles in order to study the flow field distribution within DCHERs and predict their performance. The model is extensively validated with experimental data obtained under various working conditions with discrepancies of ±10 % and ±9 % for outlet air temperature and humidity, respectively. The model provides essential information on local temperature and humidity distributions, and key insights on the transient heat and mass transfer phenomena. Further, a general design guideline for the performance enhancement of DCHERs is established. Key results from the study reveal that the air RH varies within 15 % along the DCHER’s flow channel. Thick desiccant coating, high inlet air relative humidity, and long air-desiccant contact time markedly improve latent energy effectiveness to a maximum of 0.51. Lastly, employing the proposed guideline to design the heat exchanger with optimal tube configurations yields a 23 % enhancement of latent energy effectiveness.

Numerical and experimental study of the transient conjugate heat transfer and fluid flow of passive residual heat removal heat exchanger

The transient heat transfer performance of passive residual heat removal heat exchanger (PRHR HX) with C-tube bundle directly influences nuclear reactor safety under accident condition. This paper presents an investigation of transient conjugate heat transfer and fluid flow phenomena and characteristics of scaled-down single C-tube submerged in water tank under atmospheric pressure at the initial period before saturation pool boiling occurs. Empirical calculation model, experimental setup and numerical model were established separately, and both numerical and experimental studies were carried out. The distributions of tube outlet temperature and water temperatures at horizontal and vertical monitoring points in tank were obtained. Detailed comparisons of the results show good agreements. Also, the change tendencies of heat flux, heat transfer coefficients and heat load were analyzed. The fluctuation of heat flux along normalized length from tube inlet firstly increases but finally tends to keep stable with the change of temperature difference and transformation of heat transfer mechanism. The heat transfer coefficient inside tube shows a slight downtrend along normalized length from tube inlet, but the heat transfer coefficient outside tube shows obvious change in the transient process. The total heat load continuously decreases with time. Furthermore, the volume fraction of vapor is relatively low during the subcooled boiling period. The two-phase flow enhances the agitation in water tank and the fluid velocity near tube outside wall is much larger.

Implementation of fiber optic temperature sensors in quenching heat transfer analysis

The minimum film boiling temperature (Tmin) is one of the primary factors that influence the efficiency of rewetting of nuclear fuel rods after a loss-of-coolant accident (LOCA) in a light water reactor. In order to properly study the mechanisms behind the transient heat transfer phenomena that occurs during quenching, experimentation must be done using the most high-fidelity, high-resolution instrumentation available. In past experiments, thermocouples have been used to measure the temperature change in fuel rod simulators during quenching. However, these devices do not provide a high enough spatial resolution to obtain a full measurement of the temperature change throughout the rods. In order to study the entire test length during quenching, optical fiber temperature sensors are used and compared to the thermocouples. These fiber sensors have the capability of measuring every 0.65 mm with a similar degree of accuracy to the thermocouple measurements. By utilizing optical fiber sensors, the effects of cladding material, liquid subcooling, and surface roughness on the quenching temperature of fuel rod simulators is studied. In addition, the effect of axial heat conduction of the quenching temperature is also made possible to examine using fiber optic sensors. By better understanding the factors that influence quenching, one can properly study the effect of cladding surface properties on the safety of nuclear reactors.

Method of research for solar cookers performance characteristics- analysis and comparison

Cooking is one of the most common activity in day-to-day life of every woman. In rural areas the transportation of fuel is major problem and the increasing demand of energy for cooking applications is gaining importance and various investigations are being carried out for performance enhancement of the solar cooker. The box-type solar cooker has a complex thermal analysis due to the transient heat transfer phenomenon involved in three dimensions. A comparison of the standard correlation available are analysed for accuracy of predicted results with experimental data. The investigation involves the experimental determination of the parameters viz. wind heat transfer coefficient, side and bottom loss coefficient, inner and outer glass temperature. The extensive data is analysed with that of standard correlations and the significance of the experimental data is checked. Analysis found to have deviation of 3%-20% in experimental and correlation data, which indicates that for accuracy of performance analysis the studied parameters should be determined experimentally.

Enhancement of solar chimney performance by passive vortex generator

According to the global consensus on mitigation of greenhouse gas and human footprint as well as improvement of industrial sustainability by the exploitation of renewable energy, the mission of this study dedicated to the performance enhancement of planar solar chimneys in converting much more solar radiation into air enthalpy by vortex generation technique. For this purpose, a passive vortex generator (VG) as an inexpensive but effective approach is employed for the enhancement of heat transfer. The well-developed commercial software of COMSOL Multiphysics was used for two-way modeling and simulation of sophisticated fluid-solid interaction in transient turbulent natural convection heat transfer phenomena. An extensive comparison has been made between two configurations of solar chimney namely with and without VG. According to the numerical results, the natural vibration of elastic VG induces moving vortices into the flow field which is also responsible for the enhancement of natural convection heat transfer by mixing and reducing the thermal boundary layer. Moreover, the VG significantly improves ΔToutlet up to 300% and also decreases the temperature of the absorber surface more than 50% which is under constant heat flux. Both of these factors lead to higher performance for natural air heater.

Influences of deposition strategies on thermo-mechanical characteristics of a multilayer part deposited by a wire feeding type DED process

Variation in process parameters and deposition strategies has produced different thermo-mechanical characteristics in a part fabricated by a wire feeding type directed energy deposition (DED) process. The purpose of this paper is to select a proper deposition strategy to fabricate a straight wall part using a wire feeding type DED process. The deposition strategies include different deposition directions, deposition patterns and interpass time. In order to investigate the influences of deposition strategies on thermo-mechanical characteristics of fabricated part, three-dimensional finite element analyses (FEAs) have been proposed. A volumetric heat flux has been applied to simulate the transient heat transfer phenomena of the DED process. Influences of different deposition strategies on thermo-mechanical characteristics of fabricated part have been discussed. From the result of residual stress distribution, an appropriate deposition strategy has been selected for the multi-layer deposition. Results of FEAs have been verified through comparison of vertical displacements between estimated results from FEAs and those of experiments.

Heat transfer to water near the critical point: evaluation of the ATHLET thermal-hydraulic system code

Abstract The heat transfer coefficient is an essential measure in the predesign of supercritical water-cooled reactors (SCWRs). At supercritical pressures, three distinct heat transfer modes exist: normal, improved, and deteriorated. The heat transfer behavior of supercritical water in the pseudo-critical range is different from that of single-phase fluids in the subcritical range. These heat transfer modes differ from those of single-phase flow at subcritical pressures, resulting in an unusual behavior of the heat transfer coefficients. Moreover, during accidental scenarios, when the operating pressure is reduced from supercritical to subcritical conditions, a boiling crisis may occur. During pressure reduction, temporary phenomena such as superheating of the cladding temperature can endanger the safe operation of SCWRs. In order to analyze operational and accidental scenarios of SCWRs, thermal-hydraulic system codes such as ATHLET are applied. However, the prediction capabilities of thermal-hydraulic system codes rely on a comprehensive validation work based on experimental data. This study presents an extensive analysis of the applicability of ATHLET at the near-critical pressure range. ATHLET is assessed against the LESHP-database and two trans-critical transient experiments. At supercritical pressures, the heat transfer coefficient correlations are evaluated with regard to their prediction accuracy and numerical problems including the “multiple solutions problems”. The trans-critical transient experiments are used to test the prediction capability of ATHLET with respect to transient heat transfer phenomena including critical heat flux, film boiling and return to nucleate boiling.

Analysis of a latent heat thermal energy storage unit with metal foam insert in both the HTF and PCM sides

Latent heat thermal energy storage (LHTES) systems using phase change material (PCM) have received significant research attention in numerous engineering fields. The transient heat transfer phenomenon inside a vertical LHTES unit is numerically investigated, with the paraffin as the PCM and water as the heat transfer fluid (HTF). As a performance enhancement technique, the metal foam insert is applied in both the HTF and PCM sides. The conjugate thermal model for the HTF/foam-wall-PCM/foam system is built, considering the non-equilibrium effect between the solid and fluid phases with two-temperature energy equation and the natural convection inside the PCM with Boussinesq approximation. The enthalpy-based method is employed to account for the solid-liquid phase change problem. The overall performance is compared with other three cases: no foam insert, foam insert in HTF side and foam insert in PCM side. Besides, parametric study is also conducted on the melting features, including the foam structural parameters and inlet conditions of HTF. The results show that foam insert in both sides accelerates the PCM melting effectively. The foam porosity and HTF inlet temperature play important roles in the overall heat transfer, whereas the pore density and HTF inlet velocity have limited effects on the melting rate. The findings can provide referential information for the design of a LHTES system.

On the comparison between computational fluid dynamics (CFD) and lumped capacitance modeling for the simulation of transient heat transfer in solar dryers

In this paper transient heat transfer in a solar food drier is modeled by using both computational fluid dynamics (CFD) and lumped capacitance modeling techniques. The CFD model is used to simulate the air flow and transient heat transfer within the dryer. A lumped capacitance model, developed from energy exchanges between various components of the drying chamber, is extensively compared with the results of the CFD model. The two models predict both a 40 °C rise in steady-state temperature within the drying chamber. Before reaching this temperature, there is about 8 °C RMS deviation between the transient temperature prediction of the two models. This deviation is mainly caused by the heat transfer coefficient around the food rack shelfs. Fitting these coefficients give rise to a minimal RMS deviation between the two models of 1.8 °C and 0.9 °C on the lower and upper rack shelfs respectively. The transient heat transfer phenomenon from CFD modeling also showed that the spatial distribution of temperature after 30 min is uniform thus validating the very assumption of lumped capacitance modeling. This demonstrates that the lumped capacitance model can supplant the computationally demanding CFD modeling technique in predicting transient heat transfer phenomenon in solar food dryers.

Heat Transfer Enhancement in Transformers by Optimizing Fin Designs and Using Nanofluids

In this paper, we have simulated different fin geometries at different flow intensities to study the optimum design for better flow and heat transfer characteristics to be used in transformers’ cooling. We use energy density of oil (pressure) as flow intensity parameter being a compressible fluid which is dependent on temperature variation. We observe direct proportionality of shear stresses (pressure drop) with flow intensity. Pressure drop is dominant in rectangular fins with higher height-to-width ratio (h/w), and it decreases sharply for lower h / w ratio especially at bends, whereas it is significantly better in conic-shaped fins especially at $$a\ge $$ 1.2. We observe an inverse proportionality of temperature drop with the flow intensity due to transient heat transfer phenomenon. We observe smooth temperature drop for conic-shaped fins. We have also investigated oil-based alumina nanofluids (in different wt/V ratios) as coolant for heat transfer enhancement in transformers. It is observed experimentally that dielectric strength improves with oil-based nanofluids. We obtain about 8.67% better results by adding 0.08% particles in oil. Comparative analysis with previous works shows that alumina-based nanofluids have better results than others. Still, there is a lot of work to be done in their use at commercial level due to their short durability.

Transient Heat Transfer Characteristics of Supercritical Fluid during Rapid Depressurization Process

A combination of experimental studies and numerical simulations is employed to probe into the transient heat transfer phenomenon during rapid depressurization of supercritical fluid in a cylindrical vessel connected to atmosphere via a sudden valve on the top. The effects of initial conditions on heat transfer characteristics are studied, and the morphological characteristics of supercritical fluid are analyzed. Results show: (i) there is a strong re-circulation inside the supercritical fluid with a sudden pressure drop. The heat transfer process is divided into four stages: the initial depressurization stage, the rapid evaporation stage, the nucleate boiling stage and the surface evaporation stage; (ii) under the subcritical state, a higher temperature contributes to a faster temperature drop rate and a shorter fast evaporation time. Moreover, a higher initial pressure leads to a lower equilibrium temperature and a shorter time it takes to reach the equilibrium temperature; (iii) differences are presented in terms of the effect of initial pressure in subcritical and supercritical zones on temperature drop rate with a turn near the critical point. A higher initial pressure gives rise to a higher equilibrium temperature, a slower temperature drop rate and the gradual increasing fast evaporation time; (iv) the experimental data are in good agreement with the calculated results, with a maximum error of 10%.

Modeling of transient characteristics of an air source heat pump with vapor injection during reverse-cycle defrosting

A thorough literature review indicates that there is a dearth of modeling studies on the defrosting dynamics of air source heat pumps (ASHPs) due to the complex underlying physics and numerous computation details. In an effort to bridge the research gap, this paper first presents a five-stage hot-gas defrost model which is incorporated into a distributed-parameter heat exchange model integrated with a detailed frost growth model proposed in Qiao et al. (2017). Numerical treatments are then proposed to smooth the discontinuous transition between different defrosting stages, resulting in a significant improvement in numerical robustness. The developed models and the component models described in Qiao et al. (2015a) are applied to construct the system model of an air source flash tank vapor injection (FTVI) heat pump. A dynamic simulation is conducted to explore the transient fluid flow and heat transfer phenomena of the system during the reverse-cycle defrosting (RCD). A detailed description of the resulting physical processes is provided. The simulation results indicate that the energy used to melt the frost accounts for 17.7% of the total energy supply from the refrigerant flow, which is consistent with published experimental data for the efficiency of a typical defrosting process.

A preliminary heat transfer analysis of pulse detonation engines

Abstract Detonation engines offer higher theoretical thermal efficiencies as compared to their deflagration-based counterparts. Simultaneous pressure gain during heat addition through the detonation wave provides the superiority to the Zeldovich-von Neumann-Dring (ZND) cycle which is used to define the thermodynamic process of detonation engines. Combustion temperatures can rise as high as 3000 K across the detonation wave. The continuous exposure to such elevated temperature may risk the integrity of the structural components of the engines. Consequently, heat management of the detonation engines highlights an important parameter on the construction of a demonstrator. In order to be able to design an appropriate cooling system, both for pulse detonation and rotating detonation engines (PDE & RDE), an accurate estimation of the heat load stands as an essential prerequisite. Hence, a preliminary numerical study of the heat transfer on a pulse detonation engine model was conducted to quantify the heat load. Conservation equations for deflagration-to-detonation transition (DDT) in detonation engines were solved through open source fluid dynamics solver OpenFOAM equipped with ddtFoam module. Reactive flow field of premixed mixtures (Hydrogen-air) was modeled with a URANS second-order approximate Riemann solver equipped with Weller combustion model, and Arrhenius equations of O’Connaire reaction scheme for Hydrogen-air detonation. Multiple boundary conditions were tested to achieve the most appropriate model. Natural convection over the lateral combustor peripheries found to be the most realistic boundary condition for the problem. In order to observe cooling process better, PDE tubes in different length were also simulated. Finally, the transient heat transfer phenomenon across the pulse detonation tube is documented for various conditions investigated.

Design and performance evaluation of a multilayer fixed-bed binder-free desiccant dehumidifier for hybrid air-conditioning systems: Part II – Theoretical analysis

The interdependent physics involved in the transport, adsorption, and thermal phenomena within desiccant-based dehumidifiers makes the construction of theoretical models essential to investigate intrinsic mechanisms. In Part II of this series, a computational model is established that increases an understanding of transient mass and heat transfer phenomena in a multilayer fixed-bed binder-free desiccant dehumidifier (MFBDD). A two-dimensional (2D) model incorporating the principles of momentum, mass and heat conservation is constructed. A modification of Darcy’s law is employed to evaluate the frictional resistance due to the presence of Micro Sphere Gel (M. S. Gel) beads in the desiccant bed. In the study, the moisture adsorption characteristics of M. S. Gel are theoretically described by employing a Linear Driving Force (LDF) model and a modified Langmuir-Sips model (based on the local relative humidity RH) owing to the unique sigmoid-shape of its adsorption isotherm at various temperature levels. The energy conservation is considered to estimate phase transition latent heat because of the moisture adsorption and heat loss to surroundings through the metal frames at different process air conditions. The validity of the model is tested by comparing the theoretical predictions with experimental observations. The results indicate that the predicted adsorbed water within the MFBDD, dehumidification capacity, temperature increase, and pressure decrease based on the simulation are in quantitative agreement with the experimental results obtained in Part I. The experimentally observed transient translocation of maximum temperature inside the M. S. Gel bed is theoretically reproduced and analyzed.

Experimental investigation on transient heat transfer in 2 × 2 bundle during depressurization from supercritical pressure

The experimental investigation on transient heat transfer of supercritical water during depressurization in 2×2 bundle with wire wraps has been performed on the SWAMUP-II co-constructed by CGNPC and SJTU. The test section consists of two channels separated by a square steel assembly box with round corners. Water flows downward in the first channel and then turns upward in the second channel to cool the 2×2 bundle with wire wraps installed inside the assembly box. The bundle consists of four heater rods of 10mm in O.D. arranged with pitch-to-diameter ratio of 1.18. Experimental parameter ranges cover pressure from 16 to 26MPa, mass flux from 850 to 1450kg/m2s, heat flux from 250 to 650kW/m2, inlet fluid temperature from 345 to 365°C, and depressurization rate of 1 and 2MPa/min. The experimental data are obtained and heat transfer characteristics during depressurization from supercritical to sub-critical conditions are discussed. The fraction of heat transfer from inner channel to outer channel is lower than 15% under the test conditions. Four different transient heat transfer phenomena during depressurization are observed. The boiling crisis is likely to occur in SCWR during depressurization as the operating condition of SCWR is compared with the test condition. The boiling crisis is less likely to occur with the increase of mass flux, and more likely to occur with the increase of heat flux or inlet fluid temperature. The wall temperature increment during boiling crisis is smaller with higher mass flux, and larger with higher heat flux or inlet fluid temperature. The depressurization rate affects the transient heat transfer itself trivially.

Influence of Nozzle-to-Surface Distance Ratio and Reynolds Number Variation on Hemispherical Tempered Glass Strength and Quench Time

The quenching step in a glass tempering process is a transient heat transfer phenomenon which is governed by several parameters – Reynolds number (Re) and the nozzle diameter-to-surface distance ratio (H/D) . In this research, the effect of such parameters on the strength and quench time of hemispherical tempered glass are to be analyzed. The quenching process will use the impinging jets quench method, with an equilateral – staggered nozzle arrangements. The process is performed in an ambient air of 60 o C and with a nozzle pitch and diameter of 27 mm and 4 mm respectively. The study applies variations of Reynolds number: 2300, 10000, 20000, 30000, 40000, 50000, 60000, 70000, 80000, and 87000, and H/D: 2, 6, 9, and 12 . These variations are used to construct and solve a mathematical model, to obtain temperature distribution contours. The contours are then transformed into stress distribution graphs. From these steps, it is found that the tempered glass strength increases and the quench time decrease along with the increase of Re and the decrease of H/D. It is also found that the allowable range of operation is between Re = 8000 – 25000 for H/D =2 and Re = 8000 – 30000 for H/D = 6, 9, and 12.

An efficient 1D numerical model adapted to the study of transient convecto-diffusive heat and mass transfer in directional solidification

We combine an effective diffusivity model with a numerical approach initially proposed by Meyer (1981) to simulate transient heat and mass transfer phenomena in a directionally solidifying Sn-Bi rod. This particularly efficient 1D numerical model is light enough to be used within the frame of optimization methods at reasonable numerical cost. This approach is tested against reference in situ measurements obtained under microgravity conditions during the Mephisto program. We simulate the final homogenization transient of several experimental runs with different pulling velocities. The solid/liquid interface temperature evolution with time is extracted from the simulations and compared with that obtained by Seebeck in line measurements. After optimization of the model the observed discrepancy between the simulated and measured data is less than 1.5%. This validates both the proposed very efficient 1D numerical approach and the consistency of the set of thermophysical parameters values for dilute Sn-Bi alloys.

Comparison of the damage induced by thermal shock in hardmetals and cermets

Thermal shock damage has been investigated in a WC-6wt.%Co hardmetal and in a TiMoCN-26wt.%Ni cermet by measuring their loss of bending strength after repeated quenching in water. The behavior of the hardmetal is significantly better than that of the cermet, showing a less abrupt loss of strength as quenching temperature step increases. Experimental values are in good agreement with R parameter predictions, especially for the cermet. Biot numbers are low enough to neglect the effect of transient heat transfer phenomena. It is concluded that a low thermal expansion coefficient combined with high strength and toughness are critical parameters to ensure an optimum thermal shock resistance.