Direct Heat Transfer Research Articles

Water Balance Test, Analysis and Application of the Wet Cooling Tower

This study aims to conduct a water loss test and analysis of water used in wet cooling towers. The measuring tools and methods of the liquid level in the collecting basin are clearly guided by establishing the water balance equation. The U-type liquidometer measuring system is used to measure the liquid level of specific open container with a large surface area. The minimum resolution of the U-type liquidometer is 1 mm. The collecting basin area is 375 m2, and the detected water volume is 0.375 m3 mm-1 liquid level. The measurement error caused by water surface fluctuation is overcame. The accuracy of the liquid level data is further improved. Under the water balance test condition, the water loss at the outlet of tower is between 26.25 m3 h-1 and 49.36 m3 h-1, and the loss of exceeding standard is 22.62 m3. This work determined the water balance condition of the wet cooling tower equipment and conducted a detailed analysis of water loss and the water conservation of the wet tower. Moreover, this study promote equipment manufacturing related to the mechanical draft cooling tower industry to the direction of water conservation, energy conservation and environmental protection to provide support. This approach is also widely used in the other air and water systems direct mixing heat transfer equipment.

Performance enhancement of a maple leaf-shaped latent heat energy storage unit by adding nanoparticles and leaf vein fins

In this paper, the solidification process of a phase change material (PCM) in a maple leaf-shaped Latent Heat Energy Storage (LHES) system applying fins in the form of leaf veins and nanoparticles as direct and indirect heat transfer enhancement methods is investigated numerically. These systems are used to balance energy supply and need. In this research, both methods are used to solve the problem of pure PCM, ie low thermal conductivity. The innovative fins used in this work are geometrically optimized using the Response Surface Method (RSM). The goal of optimization is accelerating the solidification process and reducing the full solidification time (FST). In the second method, nano-enhanced phase change material (NEPCM) is made by adding MWCNT nanoparticles to PCM (water). Standard Galerkin Finite Element Method (SGFEM), along with adaptive grid refinement, has been used to simulate the solidification process. The two methods were examined separately and simultaneously (Hybrid method), and the results were compared. Although the hybrid use of the two aforementioned methods is excellent with a 56.9% reduction in FST if the two methods are used separately, the method of using optimized fins in the form of leaf veins with a 42.6% reduction in FST is much more effective than making NEPCM with a 25.5% reduction in FST.

Web crippling of slotted perforated Cold-Formed Steel channels under EOF load case: Simulation and design

The development of new Cold-Formed Steel (CFS) channels with staggered slotted perforations has led to advances in improved thermal efficiency of buildings. These new generations of CFS channels reduce the thermal bridging effect interrupting the direct heat transfer across the web. However, the integration of these staggered perforations creates challenges in terms of reduced structural capacity. It is therefore vital to study the structural behaviour under various loading scenarios. Therefore, the web crippling performance of staggered slotted perforated channels under End-One-Flange (EOF) loading condition and flanges unfastened to bearing plate was investigated in the present paper. Finite Element (FE) models were developed to capture the web crippling strength and failure mechanism of these staggered slotted perforated channels. The validity of the FE modelling techniques was ensured by comparing the web crippling experimental results of CFS channels with solid and perforated webs under the EOF loading. Upon validation, an extensive parametric study comprising 360 FE models was then performed with the aim of (i) examining the effect of staggered slotted configurations and (ii) the corresponding degree of web crippling strength reduction. The results provided a direct mean of notable web crippling strength reduction up to 74%. The numerically derived data points were used to develop a reduction factor based new design equation, which can directly be applied to predictive equations of web crippling. The proposed approach yields more accurate and consistent strength predictions and improves the understanding of CFS channels with staggered slotted perforations.

Microwave directional pyrolysis and heat transfer mechanisms based on multiphysics field stimulation: Design porous biochar structure via controlling hotspots formation

The microwave pyrolysis of biomass and the directional design of biochar structure are examined in this study by exploring the internal connection between energy conversion and biochar structure evolution. Through the optimization of process parameters (microwave power and sample volume), the average utilization efficiency of microwave energy increased from 11.96% to 32.84%, and the specific surface area of the biochar increased from 453.89 m2/g to 638.82 m2/g. According to the simulation results of the COMSOL Multiphysics software, the distribution of the temperature field was closely related to the evolution of the microwave electric field intensity. The local high temperature areas (hotspots) were primarily located in the center of the biochar, and the temperature distribution gradually decreased from the inside to the outside. The hotspots contributed to the promotion of the conversion of amorphous carbon to graphite carbon. Meanwhile, the graphite carbon with a stronger microwave absorption capacity formed a local hotspot and promoted the growth of the pore structure. The rich pore structure of the biochar facilitated the multiple reflection of microwave energy and prolonged the microwave residence time. Graphite carbon can absorb microwave energy and convert it into heat energy to improve the utilization efficiency of microwave energy. This paper combined the experimental and simulation results to reveal the heat transfer mechanism in the microwave pyrolysis process, which provided a new perspective for the directional reduction of microwave pyrolysis energy consumption and the optimization of the microstructure of biochar.

Simultaneous design of heat integrated water allocation networks considering all possible splitters and mixers

Heat integrated water allocation networks (HIWAN) can be recognized as a nexus between water and energy within the process industry. Water and energy are so tightly related in this kind of networks that water saving options may cause more energy consumption or vice versa. Various mathematical programming methods have been proposed to simultaneous optimize HIWAN. However, this problem is believed to be difficult to solve. Compared to the traditional HEN, the direct heat transfer of stream mixing is more flexible in this problem and the actual number of heat exchangers can be much smaller than the traditional HEN. This paper takes advantage of these two features to build up a novel superstructure to simultaneously design HIWAN. Stream splitters and mixers are considered for every water-using operations and heat exchange units, so that possibilities of stream mixing can be fully explored and there are more series and parallel modes between heat exchangers than that of stage-wise superstructure. The proposed model is solved under given different number of heat exchange units. A small to large iteration strategy is introduced to find the best solutions. Applying this proposed approach, the better results are obtained in large-scale problems compared with the previous papers.

Three-dimensional numerical study of the directional heat transfer in an L-shaped carbon/carbon composite thermal protection system

Designing an accurate and efficient heat protection system for the area around the high-temperature stagnation point on supersonic aircraft is still an important topic of research. In the present study, a three-dimensional high thermal conductivity carbon/carbon (C/C) composite thermal protection system embedded with L-shaped carbon fiber bundles for directional heat transfer is proposed as a high-efficiency thermal protection design. A multi-scale method, which couples the finite volume method (FVM) and lattice Boltzmann method (LBM), is developed to investigate the directional heat transfer in the proposed structure. The FVM is used to calculate the heat radiation information, which is needed to solve the energy equation with the LBM. Further, the failure temperature of the proposed thermal protection structure is defined. The effects of the porosity, carbon fiber bundle and pore diameter on the directional heat transfer of the L-shaped C/C composite thermal protection system are investigated in detail. The results show that the effective thermal conductivity of the proposed thermal protection system increases with increasing temperature and carbon fiber bundle diameter. It decreases with increasing porosity when the temperature is below 1000 °C. There exists the competitive relationship between the pore heat radiation and carbon fiber bundle thermal conductivity. An increased porosity results in a decrease in the failure temperature of the proposed thermal protection structure, while increasing the carbon fiber bundle diameter can increase the failure temperature. These findings can provide some new insights for designing a high-performance C/C composite thermal protection system.

Numerical modeling of radiation heat transfer from combusting droplets for a sodium fire analysis

Heat radiation is one of dominant heat transfer processes during a sodium fire event which is a concern in sodium-cooled fast reactor plants. The multi-dimensional sodium fire analysis code AQUA-SF employs the 6-flux gas radiation model to simulate radiation heat transfer in atmospheric gas and to structural walls with the consideration of the radiation effects of suspended sodium by-products as aerosols. Here, this original model does not calculate radiation heat transfer from combusting droplets of sprayed sodium. All the combustion heat transports to atmospheric gas surrounding the combusting droplet except for the heat distributed to the droplet in this model. However, some of combusting heat should transport to structural walls directly by heat radiation in actuality. Due to this modeling, atmospheric temperature and pressure are overestimated and on the other hand, temperature of structural walls is underestimated. Therefore, this study aims to model radiation heat transfer from combusting droplets numerically. In the modeling, radiation energy transport on the combusting area around a sodium droplet is formulated considering emission, absorption and scattering as interaction with surrounding gas radiation. Radiation energy from the combusting droplets is added to the source term of the radiation transport equation in the 6-flux gas radiation model. Direct radiation heat transfer from combusting droplets can be simulated by this improved model. The improved model is tested through the verification analyses of single droplet combustion and the benchmark analysis on the upward sodium spray combustion experiment. The results of the test analyses show increase in heat transfer to the walls due to the droplet radiation. As the result, the sodium fire analysis becomes more reasonably by the improved model.

Conjugate Heat Transfer in a Thin Microchannel Filled with a Porous Medium

We study numerically the conjugate heat transfer between the walls of a parallel plate microchannel and a homogeneous porous medium fully saturated with a liquid that is found in motion due to an external pressure gradient. The origin of this problem is caused by a uniform heat flux imposed at the external surfaces of the walls of the microchannel that have a finite thermal conductivity. In this manner, the competition and heat transfer mechanisms between both regions characterized by the thermal resistances, conduct to a conjugate formulation that originates a dimensionless conjugate parameter . This parameter measures the ratio of both thermal resistances, and, for large values of this parameter, the longitudinal heat conduction effects in the walls are very important and suffer significant deviations when compared with the case with finite values for this parameter. The dimensionless governing equations for both regions are established with the corresponding boundary conditions, and the numerical results show that the aspect ratios of both regions, controlled through the dimensionless parameters and , play an important role in distinguishing the presence of the longitudinal heat conduction effects in the walls. For instance, if the ratios and , the longitudinal effects of heat transfer are very important in the walls of the microchannel, whereas in the porous matrix there are effects of heat transfer in both directions, whereas if only the longitudinal conduction effects are significant for both regions.

A Modeling Study of the Respiratory System for an Early Intervention of COVID-19 and Its Transmission

Background: One of the most challenging aspects of coronavirus disease 2019 (COVID-19) is that a newly infected individual shows diagnosable symptoms, such as body temperature (Tb) rise, several days after contracting the disease. In the early phase of infection (i.e., incubation period), an undiagnosed and unaware individual can spread the virus to others. The fastest and most efficient route of COVID-19 transmission is the respiratory system. Therefore, developing a model of the respiratory system to predict changes in the lung performance upon COVID-19 infection is useful for early diagnosis and intervention during the incubation period. Objectives: This modeling study aimed to evaluate the respiratory system to present an early intervention for COVID-19 and its transmission. Methods: A simple model was developed by performing mass and energy balances on the lungs; it was simulated by the Aspen HYSYS chemical process simulator. Results: To compensate for the virus-infected lung inefficiency, the O2 concentration increased in the exhaled air at the cost of decreased CO2 concentration. Contrary to previous findings on the reduced stability of coronavirus in hot and humid environments, it was found that very hot and humid environments promote the viral transmission rate because of the direct heat transfer to the body via respiration and condensation of water vapor that may cause infection in the respiratory tract. Conclusions: Our model revealed that measurement of O2 or CO2 composition of exhaled gas, using a non-invasive and inexpensive device at home, allows for the early diagnosis of infection and its prevention. This study also aimed to highlight the actual effects of high temperature and high relative humidity (RH) on increasing the virus transmission rates, as opposed to the generally accepted hypothesis of decreased coronavirus stability under these conditions.

A mathematical model for through-air drying process of yarn bobbins

In this study, a mathematical model has been developed to simulate the through-air drying process of yarn bobbins. For this purpose, experimental data was obtained in a prototype experimental set up by passing pressurized hot air through the wool yarn bobbins. First of all, the physical phenomenon expressing the drying process has been reduced to the heat transfer problem and then a mathematical model has been written for the drying process which also includes the convective term. Using the experimental data, the coefficient included in the mathematical model was found by the extremal method. So, an inverse problem was solved. The accuracy of the model was checked by comparing with the experimentally obtained temperature values after solving a direct heat transfer problem in the given conditions. Good correlation between the obtained model results and the experimental results shows the accuracy of the mathematical model.

Vapor chamber with two-layer liquid supply evaporator wick for high-heat-flux devices

Modern electronic devices with high heat flux increasingly exceed the capability of thermal management method. Two-phase thermal dissipation devices such as vapor chambers offer effective method for high heat flux cooling. Various new structural designs of vapor chambers are used as effective thermal control methods for high heat flux devices. However, the optimization criteria for the capillary wick structures are different. The combinations between the structural surface parameters and pore size of the capillary wick are different, while their influence on the heat transfer performance need to be studied. In this work, it is reported that the fabrication and thermal performance of vapor chambers with multi-layer sintered wicks and multi-artery wick design for liquid supply. We numerically and experimentally characterize the two types of sintered multi-artery wick for replenishing liquid and escaping vapor effectively with high heat flux. Three kinds of combinations of different copper particle sizes were applied to the sintered wicks in order to balance the liquid collecting rate and flow resistance. The copper particles of large particle size (60 μm/100 μm/150 μm) in the upper layer wick and copper particles of small particle size (30 μm/50 μm/100 μm) the lower layer wick were applied in the vapor chambers. In the present study, as an evaluation criteria for capillary wick designs, the relationship between surface porosity, surface area and critical heat flux was analyzed to obtain the experimental optimization of structural design and pore size combination as 50/100 μm. It was found that with similar multi-artery structure wicks, higher surface area porosity can effectively increase the critical heat flux of the vapor chambers. Judging from the experiment results, the minimum through-plane direction heat transfer resistance of the vapor chamber is 0.1426 K/W, the minimum in-plane direction heat transfer resistance is 0.016 K/W, while the critical heat flux of the vapor chamber is 262 W/cm2.

Three-dimensional pore-scale study of the directional heat transfer in a high thermal conductivity carbon/carbon composite protection system

High-efficiency thermal protection is becoming a critical design criterion for supersonic aircraft. Currently, there is significant research on accurately and efficiently designing heat protection systems, particularly at the high temperature stagnation point region of supersonic aircraft. In this work, a three-dimensional high thermal conductivity carbon/carbon composite thermal protection system with a directional heat transfer function is proposed to realize a high-efficiency thermal protection design in the stagnation point region. A comprehensive method that combines the lattice Boltzmann method (LBM) and finite volume method (FVM) was designed to investigate the heat transfer process in the proposed structure at the pore scale. The FVM was applied to calculate the heat radiative information, which was required to solve the energy equation with the LBM. A failure temperature is defined to quantitatively describe how well the heat transfer occurs in the designed direction in the proposed structure. The effects of the temperature of the stagnation point, thermal conductivity of the carbon fibers, carbon fiber diameters, and position of the carbon fibers on the heat transfer in the proposed structure were studied in detail. The results show that the proposed comprehensive method can predict the heat transfer in the proposed structure accurately. A competitive relationship exists between the directional heat conduction and the radiation heat transfer. The radiation heat transfer became more dominating as the temperature increased. The failure temperature for the directional heat transfer in the proposed structure increased with the thermal conductivity of the carbon fibers and the carbon fiber diameters, while it decreased as the distance between the position of the carbon fibers and the nose of the blunt structure increased. The above phenomena will be helpful for designing high-performance thermal protection systems with carbon/carbon composites.

Electrocaloric cooling over high device temperature span

Summary Electrocaloric cooling demonstrates direct electricity utilization and high energy efficiency, and is often proposed as an environmentally benign, solid-state alternative to the conventional vapor compression cooling technology. As a result, the fabrication of effective electrocaloric materials and engineering of electrocaloric cooling devices have attracted increasing attention in the cooling community. However, the limited material adiabatic temperature change under safe operation field poses a major challenge to practical electrocaloric cooling device development, since real-world applications usually require device temperature span over 20 K. This perspective presents recent efforts devoted to design electrocaloric cooling devices based on active heat regeneration and cascading approaches; both strategies have achieved significant device temperature lift ~10 K with adiabatic temperature change of 2~3 K. The strategies discussed in this work are broadly applicable to the design of efficient and compact cooling systems.

An application of conjugate gradient technique for determination of thermal conductivity as an inverse engineering problem

Inverse problems frequently occur when boundary conditions/thermal properties of a material are unknown, and are determined from known internal temperature measurements. A direct problem is the opposite of an inverse problem. For example, it would be the task of determining the temperature field within a material body, given boundary conditions and thermal properties. In an ill-posed, inverse problem, small variations in internal temperature measurement cause large variations in the estimated quantity such as boundary heat flux. Measured temperature variations may be due to noise, measurement error and instrumentation error, among other factors. There are various methods for solving inverse problems in materials engineering. In the paper, an inverse problem of estimating boundary heat flux for one-dimensional heat conduction is solved using the classical method of Beck and temperature dependent thermal conductivity is estimated using the more recent and advanced Conjugate Gradient method (CGM). The temperature values from the finite difference application to a direct heat transfer problem are used along with noise added to the data, in order to simulate real world, experimental conditions. The data is applied to the inverse problem of finding boundary heat flux and thermal conductivity. It is seen that Conjugate Gradient Method (CGM) along with sequential function specification method recovers the boundary heat flux and thermal conductivity quite accurately for noise levels of 1% and 2%. It is further observed that addition of future time steps stabilize the algorithm and increase accuracy of the desired results. It is also observed that CGM can recover the temperature dependent thermal conductivity quickly within few iterations, for different values of thermal conductivity function coefficients, k0 and k1. CGM is found to be fast and robust when applied to one-dimensional case and can be extended to complex geometries.

Fragmentation regimes during the thermal interaction between molten tin droplet and cooling water

A series of experiments was performed on the interaction between molten tin droplet with cooling water, and the experimental variables included initial temperature and falling height (impact velocity) of the droplet. At the initial stage of the interaction, a vapor film is formed immediately between the droplet and cooling water. Afterwards, it collapses locally owing to the Kelvin-Helmholtz instability and Rayleigh-Taylor instability. As the temperature of direct contact area between the droplet and cooling water is below the homogeneous nucleation temperature of water, the non-fragmentation regime occurs at the low-impact velocity of the droplet. The increased impact velocity of the droplet induces the sheet fragmentation which originates from the enhanced shear force acting on the droplet. As the temperature of direct contact area exceeds the homogeneous nucleation temperature of water, the honeycomb and granular fragmentation regimes are observed. The spiny structures are formed on the droplet surface by the rapid generation of local high vapor pressure in both regimes. The honeycomb fragmentation regime occurs at the low-impact velocity of the droplet. The direct heat transfer area is dramatically increased as the droplet impact velocity increases, resulting in the granular fragmentation regime. The granular fragmentation is affected by both the hydraulic factor (shear force) and the physical properties of molten tin.

Inducing directional heat transfer by enhancing directional thermal conductivity of asphalt mixtures for improving asphalt solar collectors

Low thermal conductivity makes heat conduction in asphalt pavements quite slow, restricting asphalt solar collectors. To enhance the directional thermal conductivity of asphalt mixtures and induce directional heat transfer, directional heat-induced channels are built in asphalt mixtures by using carbon fibers with high axial thermal conductivity. A finite element method is used to simulate and study the temperature field and heat flow distribution of asphalt mixtures. A parametric analysis of the carbon fiber parameters is performed to evaluate the variation in the performance of the effective thermal conductivity of asphalt mixtures. The temperature trend of asphalt mixtures is measured by an indoor irradiation experiment and compared with the simulation results. The results show that heat-induced channels have obvious heat convergence effects and can induce heat to be efficiently conducted along its axial direction. These channels can significantly improve the directional thermal conductivity of asphalt mixtures and are expected to play a heat transfer bridge between the pavement surface and the energy collection system when using them in asphalt solar collectors in the future.

Experimental and numerical investigation on heat transfer of supercritical water flowing upward in 2 × 2 rod bundles

This paper presents the investigation of heat transfer of supercritical water flowing upward in 2 × 2 rod bundles, both experimentally and numerically. The experiment conditions are as follows: pressures from 22.9 MPa to 26.1 MPa, mass fluxes ranging from 450 kg/m2/s to 1500 kg/m2/s and heat fluxes ranging from 0.4 MW/m2 to 1.5 MW/m2. Four heated-rods of 8 mm outer diameter formed 2 set of experiment sections with pitch to diameter ratio (P/D ratio) 1.18 and 1.3. Heat transfer coefficients between the heated rod and fluid were conducted by 3 different methods and the effect of spacer grids was clearly observed. The experiment results show that heat transfer to supercritical water in rod bundles is strongly affected by flow channels and spacer grids. Moreover, a large database of heat transfer in rod bundles was carried out and HTC were compared with tube and annuli with the equivalent hydraulic diameter. The phenomenon of heat transfer difference in rod bundle circumferential direction and enhancement heat transfer in downstream of spacer grids were observed and investigated. In addition, Computational Fluid Dynamics (CFD) method was applied to give a deep insight of velocity profile, spacer effect and heat transfer enhancement in this research.

Process Intensification through Directly Coupled Autothermal Operation of Chemical Reactors

Summary Autothermal operation of a chemical reactor involves coupling exothermic and endothermic chemical reactions for the purpose of thermal management without resorting to external energy sinks or sources. Often this is accomplished through regenerative or recuperative heat exchange between spatially or temporally separated exothermic and endothermic reactions. However, it is also possible to directly couple these reactions simultaneously within the same reactor volume, eliminating the heat-transfer bottleneck that characterizes much of chemical manufacture. It is not widely recognized that directly coupled autothermal operation allows dramatic process intensification. This perspective defines autothermal operation and contrasts it with conventional heat transfer for thermal management of chemical processes. The intensification and cost savings that can be achieved are quantified, and the implications to modular chemical process intensification are presented. Guidelines are proposed for designing directly coupled autothermal processes. Diverse examples are presented. Several challenges to expanding the field are critically discussed.

Thermal Viscous Fingering in Thermal Recovery Processes

Heat and fluid flow at the edge of steam chambers found in thermal recovery processes such as steam-assisted gravity drainage and cyclic steam stimulation remain unresolved. Given the multiple phases present and contrast of thermophysical properties, it remains unclear where instabilities occur within this thin, yet critical, zone of the process. In the research reported here, heat and fluid flow are examined in vertical and horizontal sections of a steam chamber to understand the differences between the two orientations by using detailed and fine-gridded thermal reservoir simulation models. The results show that heat transfer in vertical and horizontal directions are different with greater heat transfer found in the vertical orientation. In the vertical direction, heat transfer occurs with mobilized bitumen draining with subsequent steam moving into the emptied pore space. Conduction beyond the edge of the chamber dominates and heated, low viscosity bitumen fingers into cold, higher viscosity bitumen at the edge of the chamber. Relative permeability effects are observed which can interfere with enhanced oil mobility.

Polycrystalline TiO2 particles synthesized via one-step rapid heating method as electrons transfer intermediate for Rhodamine B removal

In this work, a one-step rapid heating method for the synthesis of polycrystalline TiO2 particles was demonstrated. By rapid and direct transfer of heat into the precursor solution, coupled with proper control of heating duration; polycrystalline TiO2 particles have been successfully synthesized in 45 min. TEM and XRD analyses demonstrated the polycrystalline nature of the TiO2 particles synthesized at 45 min composes of a mixture of 74% anatase phase (29.40 nm) and 26% brookite phase (21.07 nm). These polycrystalline TiO2 particles degraded 80% of Rhodamine B (RhB) after 60 min of visible light irradiation. The self-sensitization of RhB and crystal defects in polycrystalline TiO2 particles (electron donors), had produced more electrons in the conduction band of TiO2. These electrons then reduced the oxygen molecules to •O2− free radicals. Both electrons and •O2− free radicals were the main reactive species that degraded the RhB molecules as verified by scavenger tests. The promising photocatalytic performance together with the simple and low cost synthesis method, have provided a feasible alternative in the development of visible light active photocatalyst for organic dye removal.