Transient Heat Transfer Process Research Articles

MULTITHMT: A MATLAB Application for Multidimensional Transient Heat and Mass Transfer Processes

Transient heat conduction and mass transfer have many applications in industry such as heating, cooling, cooking, quenching of steels, freezing, and convective drying of vegetables or fruits. A novel, interactive, and fast MATLAB application, named MULTITHMT, is improved to solve multidimensional transient heat and mass transfer problems. Exact solutions are obtained for infinite rectangular bars, short cylinders, rectangular prisms, and spherical geometries. Instantaneous temperature and moisture content at any location in the objects are obtained and temperature and moisture content at the final time are displayed in two- and three-dimensional graphics. Quenching of steel for rectangle bars and cooking of cylindrical or rectangular prism-shaped meat are represented for transient heat transfer. Cooling of spherical commercial bronze and iron is also investigated. For transient mass transfer, convective drying of rectangular prunes, bananas of short cylinders, and spherical cornelian cherries with different operational conditions is calculated. Drying of cubes with the same shape and different moisture diffusivities is investigated. MULTITHMT is the only program that uses exact solutions to calculate multidimensional heat and mass transfer problems in the available literature. It is also the only application that can calculate the target time with a given temperature or moisture content for any specific location in the studied multidimensional objects. This application can be used for educational purposes in several engineering departments and industrial applications where transient heat and mass transfer processes are needed.

An improved dynamic analytical model and the parameter sensitivity analysis of spherical stacking latent thermal energy storage devices

Thermal energy storage technology plays an important role on improving energy usage flexibility for the end users. The spherical stacking latent thermal energy storage devices inherits the advantages of direct thermal energy usage efficiency and simplified practical applications that received widely attentions. However, due to dispersed and multi-layer structural arrangement, the non-ignorable dynamic heat transfer process poses difficulties in the device design and operation control. In this study, an improved analytical model for predicting the transient heat transfer process of spherical stacking devices is developed. After verification, the parameter sensitivity analysis of the device is conducted by using the model. The results show that the dynamic change trend of thermal resistance is distinct for the melting and the solidification process. The developed model can dynamically predict the outlet temperature of heat transfer fluid and the heat transfer efficiency in a good agreement, and the prediction error of outlet temperature during solidification can reach −2.49 %–4.70 %. The correlation between sphere radius and heat transfer rate is negative and reaches a kind of high correlation above 0.8 for solidification process. The inlet temperature and the mass flow rate of heat transfer fluid are the most influential parameters for melting process.

Transient simulation of conjugate heat transfer of kerosene with thermal oxidative and surface coking reactions at a supercritical pressure

The regenerative cooling technology in aerospace applications often involves supercritical-pressure heat transfer of a hydrocarbon fuel, in which thermal oxidative reactions of the fuel would lead to surface coke accumulation on the cooling channel wall. In this paper, the long-term transient conjugate heat transfer of kerosene has been numerically investigated in a circular mini tube at a supercritical pressure of 3 MPa. The modified chemical mechanism and kinetics of thermal oxidative and surface coking reactions of kerosene are proposed, which, with an additional surface reaction, are capable of treating large coke deposition in regions with sharp temperature gradients. Transient surface coke accumulation and its effect on heat transfer are handled by a dynamic mesh technique. A series of numerical studies are conducted to elucidate the influences of surface heat flux, dissolved oxygen mass fraction, inlet fuel mass flow rate, and coke layer thermal conductivity on the supercritical-pressure heat transfer and thermal oxidative reactions. The effects of different reaction pathways on surface coke accumulation are analyzed comprehensively, and it is revealed that the oxygen-consuming surface coke deposition is initiated only at the high fuel temperature at around 590 K. The present numerical results would enhance fundamental understanding of the long-term transient heat transfer process of a hydrocarbon fuel at supercritical pressures in the regenerative cooling applications.

Theoretical investigation of electromagnetic-thermal coupling of double-layer cylindrical concrete under microwave irradiation

Many experiments have been performed to study the heating properties of concrete under microwave irradiation. Microwave provides the non-uniform heating process, which cannot be reflected clearly through the experimental investigations. In this paper, a theoretical method is presented to investigate the electromagnetic-thermal coupling process of double-layer cylindrical concrete under microwave heating. The wave transmission and reflection were considered. An analytic solution is presented to predict transient heating process within a 3-dimensional double-layer concrete model induced by microwave heating. The inner aggregate is a microwave high loss material and the outer mortar was microwave low loss medium. Poynting theorem was employed to calculate the electric field distribution and microwave energy loss within concrete. Transient heat transfer process with an internal microwave heat source was investigated based on the classical heat transfer theory by employing integral transform technique. The results indicate that microwave heating effect depend on the concrete size, dielectric properties as well as microwave energy input. The temperature gradient was formed at the mortar-aggregate interface, which varied with the microwave heating parameters inputs. The analytical study will provide significant insight to promote the understanding of electric and temperature field in the two-layer composite concrete materials under microwave heating.

Dynamic Flow and Heat Transfer Characteristics of Uncracked Hydrocarbon Fuel under Super-Critical Pressure in the Cooling Channel of a Regeneratively Cooled Scramjet

Regeneratively cooled scramjets are successfully used as propulsion devices in hypersonic vehicles. During operation, scramjets experience acceleration. This special process causes a dynamic flow process, and heat transfer in the cooling channel commonly occurs, which may cause hazards and control difficulties for scramjets. A dynamic numerical model with a modified heat transfer coefficient calculation method was established to study the transient flow and heat transfer processes in a cooling channel. The dynamic characteristics of the flow and heat transfer under different conditions were calculated and are discussed, including the changes in the inlet fuel mass flow, heat flux, and pressure working conditions. The results indicate that the stable time of the cooling channel outlet fuel temperature is related to the rate of change in the inlet mass flow and heat flux. The stable time of the outlet fuel temperature under decreasing heat flux working conditions was approximately 12.5 s. These results summarize the dynamic flow and heat transfer characteristics, which are significant for designing cooling channels in scramjets.

Heat transfer characteristics of subsea long-distance pipeline subject to direct electrical heating

For deep-water pipelines with long tie-back distances, high external pressure, and low ambient temperature, it is a key issue to ensure flow safety by controlling the generation of hydrates and waxes through electric heating technology. In-detail analysis of the heat transfer performance of electric heating pipelines is of great engineering significance for developing the safety of pipeline flow in offshore oil-gas fields. This paper proposed an electrical heating analytical method applicable to long-distance submarine pipelines. Based on the thermal equilibrium theory and the differential equation of heat conduction, a mathematical model of heat-fluid coupling for electric heating pipes is established, involving the theoretical model of the steady-state distribution of the electric heating in the axial and radial directions. Then, the definition of the average temperature of the pipeline is introduced and combined with the two-point Hermite approximation interpolation method, and an improved lumped parameter model of the transient heat transfer of the electric heating pipeline is established. Meanwhile, the finite difference method is applied to solve the ordinary differential equation of the pipeline and the energy equation of the internal fluid. The steady-state calculation result is utilized as the initial value of the transient heat transfer process, and the model is applied to analyze the influence variation on the transient temperature response characteristics of the pipeline under different working conditions, finally determining the prediction formula of the average temperature of the fluid under normal transportation and shutdown conditions.

Conduction-convection coupled heat transfer around a hollow cylinder under different buoyancy forces

In order to clarify the effect of a buoyancy force on conduction–convection coupled heat transfer in a hollow cylinder, the flow and thermal characteristics were analyzed using an RNG k-ε turbulence model. The Reynolds number was fixed at 1.014 × 106, and the Rayleigh number varied from 1.122 × 1010 to 1.088 × 1011. Results have shown that, when considering the effect of an opposed buoyancy force, increasing the Rayleigh number has a positive impact on the rate of change and uniformity of the cylinder temperature. The temperature distributions along the axial and circumferential directions are similar for different Rayleigh numbers, but extreme values differ. Along the axial direction, the maximum temperature is obtained at the interface between the variable-diameter part and the constant-diameter part. The maximum dimensionless temperature value decreases to 0.12 when the Rayleigh number increases to 1.088 × 1011. Along the circumferential direction, the temperature distribution is affected by the buoyancy force, which results in the temperature of the upper part being higher than that of the lower part. After non-dimensionalization of the temperature and time, a correlation was proposed to illustrate the transient heat transfer process quantitatively. The standard deviation of the maximum relative temperature, representing the temperature uniformity, was also calculated. It was found that the difference in the direction of the buoyancy force made a huge difference. Compared with the opposed buoyancy force, the maximum dimensionless temperature is almost two times higher with an assisted buoyancy force. Similarly, the heat transfer coefficient with an assisted buoyancy force is half of that with an opposed buoyancy force. Overall, an assisted buoyancy force plays a negative role in terms of thermal characteristics. The flow field around the hollow cylinder was also illustrated to reveal the mechanism of the buoyancy force on magnitude and direction aspects.

A novel structure for improving the service life of a propulsion system by controlling two-phase reactive flow

Methods to control two-phase reactive flow induced by the combustion of propellant particles are important for improving the service life of mechanical systems. A novel groove structure was proposed to control the two-phase reactive flow. A two-phase flow model was utilized to predict propellant combustion in fluid fields and coupled with the transient heat transfer process. The numerical model was validated by comparing numerical and experimental results. Numerical results indicate that the groove structure effectively decreases the scouring velocity of two phases and the accumulation of particles near the barrel surface. The groove also reduces the peak temperature of the barrel surface, which contributes to less thermal erosion. Furthermore, shooting tests prove that the groove structure greatly extends the service life of the barrel. The findings of the study will be applied to improve the service life of propulsion systems.

A Neural Network-Based Method for Real-Time Inversion of Nonlinear Heat Transfer Problems

Inverse heat transfer problems are important in numerous scientific research and engineering applications. This paper proposes a network-based method utilizing the nonlinear autoregressive with exogenous inputs (NARX) neural network, which can achieve real-time identification of thermal boundary conditions for nonlinear transient heat transfer processes. With the introduction of the NARX neural network, the proposed method offers two key advantages: (1) The proposed method can obtain inversion results using only surface temperature time series. (2) The heat flux can be estimated even when the state equation of the system is unknown. The NARX neural network is trained using the Bayesian regularization algorithm with a dataset comprising exact surface temperature and heat flux data. The neural network takes current and historical surface temperature measurements as inputs to calculate the heat flux at the current time. The capability of the NARX method has been verified through numerical simulation experiments. Experimental results demonstrate that the NARX method has high precision, strong noise resistance, and broad applicability. The composition of the input data, the surface temperature measurement noise, and the boundary heat flux shape have been studied in detail for their impact on the inversion results. The NARX method is a highly competitive solution to inverse heat transfer problems.

Transient thermal analysis on pre-cooling process of LNG cryogenic corrugated hose

The pre-cooling mechanism of LNG (liquefied natural gas) cryogenic corrugated hoses is complicated due to the structural complexity of the hose and the rapid temperature changes during the process. Conventional predicting temperature drop methods for pre-cooling heat transfer mostly simplify it to a steady heat transfer analysis, which results in the transient heat transfer process under pre-cooling conditions cannot be well reflected, and the calculation results are inaccurate. Considering the turbulent flow effect caused by the corrugated contour, a mathematic model is built to capture the temperature distribution in hoses. The numerical integration method is used to calculate the average temperature of the section which varies with time and space position under the condition of pre-cooling and stable operation. Meanwhile, a numerical model is established and its results are in good agreement with the mathematic model results. Case studies show how those factors like the thickness of the cold insulation layer, the nitrogen inflow rate and the radius of the hose affect the pre-cooling process. It is concluded that there is a certain non-linear relationship between the internal temperature of the hose and the pre-cooling time, axial and radial position. In addition, the thickness of the insulation layer and the nitrogen inflow rate dominate the pre-cooling process. Some suggestions can be put forward for the optimization of the pre-cooling process. The theoretical formulas not only provide an efficient tool for quick and reliable calculations of the temperature of LNG cryogenic corrugated hose during the pre-colling process but also apply to cryogenic pipe systems under other conditions.

Experimental and parametric numerical investigation of finned heat sinks with organic and metallic phase-change materials

The use of phase change materials (PCMs) has gained much attention for applications of transient thermal management of electronic systems due to their high latent heat and ability to absorb heat near-isothermally. Because of their low thermal conductivity, PCMs are usually integrated with heat sinks to be used more efficiently. In this study a PCM-based heat sink with a generic structure of plate fins was investigated experimentally and numerically. The intentionally simple fin topology allowed focusing on the effects of various material properties rather than the commonly investigated geometry effects. Hence, two types of heat sink material, copper and aluminum, were examined, and two PCMs with similar melting temperatures but distinctly different thermal properties – a metallic alloy (Field's metal) and an organic paraffin (n-Octacosane), were used. Experimental findings allowed for validation of the numerical approach, used for a comprehensive parametric numerical study. The latter facilitated a more detailed investigation of the transient heat transfer processes, such as melting patterns and heat accumulation analyses, where the superior thermal properties of the metallic PCM manifested in more efficient latent heat accumulation, resulting in reduced system peak temperatures. It was found that systems with Field's metal were able to accommodate up to 80% of the energy in the form of latent heat, which is 10 percentage points higher than achieved using the organic paraffin. A dimensional analysis accounting for power inputs and material properties was conducted, and a generalized behavior was achieved for a normalized time in terms of Fourier and Stefan numbers, and thermal diffusivities ratio.

Quantitative evaluation of the natural convection effect on thermal conductivity measurement with transient plane source method

When measuring the thermal conductivity of liquids with transient methods, natural convection occurs inevitably and whether it can be ignored is unknown. The quantitative deviation introduced by natural convection should be revealed clearly to ensure reliable experiment. In this study, the influence of natural convection is quantitatively evaluated in measuring the thermal conductivity of liquids with transient plane source method. By numerically mimicking the transient heat transfer process within liquids, the sensor temperature responses with neglecting and considering natural convection are obtained and used to calculate the corresponding thermal conductivity. The difference between them represents the quantitative influence introduced by natural convection. The effects of sensor orientation, heating time and heating power are compared for lead-bismuth eutectic alloy, GaInSn alloy, deionized water, molten salt, glycerol and cupric sulfate. Experiments are performed to verify the simulation. It is found that for the liquids with lower Prandtl number (<10), the thermal conductivity measured by horizontally placed sensor is more precise than that by vertically placed sensor. On the contrary, the liquids with Prandtl number higher than 1000 are less affected (<1%) by natural convection. Shorter heating time and proper temperature rise magnitude can suppress the influence of natural convection to some extent.

Experimental investigations on thermal diffusivity of heterogeneous materials

The paper presents alternative methods for determining thermal diffusivity of heterogeneous materials. In the study the thermal regular regime method and the step-heating technique were applied to measure the thermal diffusivity of graphene oxide/rubber composites characterized by different kinds of basic matrix and different weight concentrations of the reduced graphene oxide nanoparticles. Thermal diffusivity plays a relevant role in the analysis of transient heat transfer processes. It is an essential material property which may be a measure of the quality of different kinds of engineering materials. Therefore, the knowledge of thermal diffusivity is important, especially for what heterogeneous materials (composites) concerns. In the present study composites with three kinds of matrix - have been studied: FKM- fluoroelastomer, NBR- acrylonitrile butadiene rubber and HNBR- hydrogenated acrylonitrile butadiene rubber. The reduced graphene oxide rGO in the form of crumpled flakes was used as filler in order to significantly influence the thermophysical properties of the composites. In the two tested experimental methods, - thermal regular regime method and step-heating technique, relatively large material samples were used. Hence, the influence of the material heterogeneity on the investigation results immaterial. The obtained results, both from thermal regular regime method and step-heating technique, show an increase of the thermal diffusivity with increasing the graphene oxide content in the composite. An uncertainty analysis of the results of applied research methods has been carried out, as well as measurements of the heat capacity of the matrix of two composites using differential scanning calorimetry to validate the thermal diffusivity results from the values of thermal conductivity and density.

Mixed convection heat transfer and transient flow of high-pressure gas in a large-scale cavity with rotating blade

Mixed convection heat transfer phenomenon, incorporating natural and forced convection process, occurs widely in natural and industrial fields. Nevertheless, few works have investigated numerically the turbulent mixed convection heat transfer within the large-scale structure under high-pressure condition. In this study, a three-dimensional numerical model of a large-scale closed cavity with two rotating blades and reactor has been built on account of the finite volume method, aiming to explore the mixed convection heat transfer and turbulent flow of internal high-pressure gas. Compressible gas with varying thermophysical properties was chosen as the heat transfer medium and the numerical model was solved by Unsteady Reynolds-Averaged Navier-Stokes (URANS). Turbulent flow and heat transfer, inside the enclosure and reactor, were compared through the variation of the Reynolds number (Re), gas pressure (P) and heating surface temperature (Th). The results signify that for given P and Th, varying Re has a consistent impact on the transient flow and heat transfer process inside the enclosure and reactor. The turbulent flow and thermal transport are enhanced as increasing the Re. When the Re = 900,000, an enhanced forced convection effect is found inside the reactor. Dissimilar impacts on the flow field and thermal transport inside the enclosure and reactor are found with changing gas pressure owing to the interaction of natural and forced convection. The flow field attenuates gradually as the increment in P, and thereby larger high-temperature zone is achieved at a smaller pressure inside the enclosure, whilst inside the reactor, faster heat conduction is attained at a bigger pressure. Changing heating surface temperature has a more significant influence on the flow field and heat transfer inside the reactor compared to the enclosure. Additionally, increasing the Re and Th has a smaller effect on the average Nusselt number (Nuavg) while a positive correlation between the Nuavg and P is obtained.

Study on the cladding path during the solidification process of multi-layer cladding of large steel ingots

Abstract Aiming at the quality problems such as segregation, porosity and shrinkage cavities that are difficult to eliminate due to the size effect of large die-cast steel ingots as large forging blanks, the idea of layered casting of large steel ingots is proposed. The transient heat transfer process and cladding path of the ingot core and cladding layer under different molten steel casting temperatures, different ingot core diameters and different ingot core preheating temperatures were studied by combining numerical simulation and thermal experiments. The research results show that the cladding path has a certain functional relationship with the diameter of the ingot core and the preheating temperature of the ingot core. Obviously, the interfacial melting rate can be significantly improved. The thermal scaling experiment was carried out on the cladding path under the condition of a casting temperature of 1,560°C and no preheating of the ingot core. The microhardness of the interface is higher than that of the clad steel ingot, and the metallurgical bond of the interface is good.

Heat Conduction and Cracking of Functionally Graded Materials Using an FDEM-Based Thermo-Mechanical Coupling Model

In this paper, the steady-state and transient heat transfer processes of functionally graded materials (FGMs) are analyzed using a coupled thermo-mechanical model in a GPU parallel multiphysics finite–discrete element software, namely MultiFracS. First, the coupled model to handle the heat transfer problem of heterogeneous materials is verified. Then, the advantages and disadvantages of FGMs and composite materials in response to thermal shock loads are compared and the results indicate that FGMs can overcome extreme environments better than composite materials. Finally, the influence of the geometric distribution characteristics of the double-edge cracks in the gradient material plate on the crack propagation is analyzed. The simulation results show that the interaction between the cracks affects the crack propagation path under the thermal load. The inclination angle and spacing of double-edge cracks greatly influence crack propagation. Specifically, a larger inclination angle and spacing can lead to a smaller crack propagation angle. The approach in this paper provides a new quantitative tool for investigating the thermal, elastic, and cracking of functionally graded materials.

Comparative analysis of insulated floating cover of water pit thermal energy storage

Water pit thermal energy storage (PTES) as a sensible heat storage technology has been applied in many places with significant success. The insulated floating cover (IFC) of water pit is the key component of the PTES thermal storage efficiency and accounts for more than half of the total cost of the entire water pit. In this paper, a one-dimensional finite element model (1D FEM) and a zero-dimensional lumped thermal system model (0D LTSM) are established for the composite IFC of PTES, and their respective results are compared with experiments respectively. LTSM results match the FEM resolution results if the LTSM is sufficiently refined by discretized each layer into several sub-systems with their own mass and thermal resistance. In this LTSM, 5 sub-systems are used in the thermal insulation layer, and 2 sub-systems are used in the upper and lower impervious barrier respectively. The results show that the LTSM can accurately describe the transient heat transfer process of the composite IFC. The results of LTSM and FEM are in good agreement with 99.2%, which is about 2.6% different from the experimental results.

Numerical Modelling of Moisture Transport Between Two Enclosures Connected by a Tube

The electronics is protected using plastic or metallic enclosures. Although, the electronics is protected by the enclosures from the surrounding environment, the moisture can still enter the enclosure via gasket, plastic enclosure walls, cable feedthroughs. The moisture existing inside enclosure may condense on PCBA or components due to a temperature changes or different temperature levels and can lead to a moisture-related failures. The temperature in the enclosure is also very dependent on the location and the heating of electronic components. Furthermore, the electronics can be mounted nearby to other heating or cooling components in the complex mechanical systems, such as water pump or water meter. The paper concerns the complex transient heat and mass transfer processes between two connected enclosures with one being warm and another cold. The objective of the paper is to develop an in-house code based on the RC approach for predicting and studying the temperature and moisture behaviour inside two connected enclosures. The developed RC model combines one-dimensional description and lumped analysis for heat and mass transport. The modelling of temperature and moisture response is carried out under non-isothermal B3 STANAG ambient conditions. Moreover, the effect of different material of warm enclosure and the heating are considered.

Preliminary measurements of transient boundary heat transfer process under supercritical pressures using pixelated phase-shifting interferometry

The boundary heat transfer dynamics of supercritical carbon dioxide (CO2) is vital to its application in solar heater, extraction, refrigeration, heat pumps, power generations, nuclear systems and many other occasions. In this study, an experiment system using pixelated-array masked phase-shifting interferometer has been established for the visualization of temperature of supercritical CO2. The laser diagnostic process has been operated with a carefully designed visualization chamber to give clear transient results of the density and temperature fields for the first time. Those field information has been used to calculate the heat transfer characteristic parameters of the supercritical CO2 fluid in different boundary conditions. The current quantitative visualization analysis by the utilization of pixelated methods is shown capable of giving transient analysis of detailed field evolutions. The results show that (1) small temperature differences are generated in supercritical boundary due to the typical large thermal capacity; (2) supercritical convection boundary is with much quicker thermal quenching than subcritical states, while the subcritical states boundary parameter show localized characteristics while supercritical cases show fluctuation phenomena; (3) the boundary fluctuations heat transfer field exist for supercritical states and the fluctuations affect the overall heat transfer behaviour and the field evolution in supercritical states.

Real-time reconstruction of temperature field for cable joints based on inverse analysis

Temperature monitoring is important for the operation management of cable joints. However, owing to the problems associated with calculation cost and parameter determination, existing research can only calculate the local temperature with limited accuracy in the operation stage. This does not enable temperature management and condition assessment. Therefore, combined with the analytical, numerical, and inverse analysis methods, a real-time reconstruction method of the 3-D temperature field for cable joints is proposed. In this study, the temperature distribution of the cable joint is analysed by the combination of the Ohm's law and heat transfer rules. The transient heat transfer process is transformed into a stable state heat transfer process under discrete-time conditions using the equivalent analysis method. The inverse problems aimed at identifying the surface heat transfer coefficient and the equivalent current are constructed, which can be solved by iterative calculation. The temperature experiment platform is then built to verify the proposed method. The reconstruction results are consistent with the experimental ones and meet the accuracy requirements of the operation analysis. Finally, the influence of current fluctuation, heat dissipation condition, and measurement accuracy on reconstruction results are analysed. The results show that the proposed method realizes the real-time reconstruction of the temperature field, which is reliable, robust, and anti-interference. It promotes the practical applications of temperature monitoring.