New Advancements in Heat and Mass Transfer: Fundamentals and Applications

Heat and mass transfer, widely involved in various energy systems, has great impacts on the safety, efficiency, as well as performance of these systems. The establishment of safe, stable, efficient and environmental operation schemes for these systems requires a thorough understanding of the heat and mass transfer characteristics of these systems. In addition, a more accurate and detailed understanding of the behavior of heat and mass transfer within energy systems is needed to establish more energy-efficient schemes in the pursuit of carbon neutrality and energy savings. Therefore, some traditional heat and mass transfer problems have been reconsidered and investigated with more advanced techniques to clarify the comprehensive heat and mass transfer characteristics in these systems. In addition, with the development of new technologies, the equipment tends to operate on the microscale or nanoscale. The heat and mass transfer behaviors within this microscale or in multiscale systems are new problems encountered in recent years as they show significant differences from previous systems and require special considerations. In this regard, this Special Issue “New Advancement in Heat and Mass Transfer: Fundamentals and Applications” was proposed to cover the new advancements in heat and mass transfer topics in different research fields.

This Special Issue includes sixteen research papers focusing on the heat and/or mass transfer problems encountered in the area of refrigeration [1,2,3], pipeline transportation [4,5,6], energy storage [7,8], laser heating [9], waste heat recovery [10], heat exchanger [11], food drying [12], solid oxide electrolysis cells [13], combustion [14], salt crystallization [15], and phase difference measurement methods for mass flow meters [16], as well as a review paper focusing on the numerical modelling of the flow and the heat transfer of microcapsule phase change slurry (MPCS). A brief introduction of each selected paper belonging to this Special Issue is provided below for ease of reference.

The research from Xia et al. [1] focused on the performance of a thermoelectric cooling device and designed a new subchannel finned heat sink for a thermoelectric refrigeration system. Their results show that this design improved the coefficient of performance (COP) of the thermoelectric refrigeration system.

Zhou et al. [2] established a closed-loop spray cooling experiment system to investigate the influence of the refrigerant charge on the spray cooling heat transfer performance. The heat transfer characteristics were investigated in steady-state, dynamic heating, and dissipating processes, providing a reference for future spray cooling system designs.

Yang et al. [3] concentrated on a transpiration cooling method for a strut, which is an important structure in the combustion chamber of hypersonic flight vehicles with respect to injecting fuel into the main flow. Since the struts work in severe thermal environments, efficient cooling is critical for safe operation. In this study, the authors quantitatively modelled the influences of coolant properties and working parameters on the cooling effect and clarified the mechanism of transpiration cooling.

Duan et al. [4] studied the gas–liquid interface evolution during liquid displacement by gases in a mobile pipeline. A mobile pipeline is the most effective and reliable means for emergency oil transfers. The pipeline is temporary, and after completing the task, the oil in the pipeline should be emptied before the mobile pipeline is removed, which is generally accomplished by displacing the oil with air. The evolution of this gas–liquid interface is very important for mobile pipeline evacuations. In this research, the authors studied the characteristics of the gas–liquid interface’s evolution during the displacement of water by air, which may provide guidance for mobile pipeline operations.

The research from Feng et al. [5] focused on gas migration in oil transmission pipelines during the commissioning process. Serious gas blockage problems often arise during the commissioning process of pipelines with large elevation differences due to the gas/air accumulation at the high point of the pipe, which causes overpressuring and vibrations of the pipeline and even accidents such as bursting. The establishment of a safe commissioning scheme requires a full understanding of the two-phase flow behavior within the pipeline. In this research, the authors established a gas–liquid replacement model for a U-shaped pipe during the commissioning process and investigated the hydraulic characteristics. This study may provide a reference for establishing safe and economic commissioning schemes for oil transmission pipelines with large elevation differences.

Liu et al. [6] examined the slug flow hydrodynamics of gas–liquid flow in a pipeline. Slug flow is widely present in transportation pipelines, and the accurate prediction of the slug flow is crucial for the proper design and operation of the pipeline. In this research, the authors developed a hydrodynamics unified model for the gas–liquid slug flow, and the results were validated by empirical data.

Lu et al. [7] aimed to improve the latent heat storage efficiency of a phase change material, Ba (OH)₂·8H₂O. Improving the latent heat storage efficiency is a focus of research in thermal storage engineering, which can be realized by enhancing the heat conductivity, improving the stabilization, and decreasing the supercooling degree. In this research, the authors prepared a composite phase change material by adding optimized nucleating agents and thermal conductivity agents to Ba(OH)₂·8H₂O resulting in improved phase change performances.

Li et al. [8] proposed a method for the thermal calculation of soil heat storage. Exploiting the heat energy stored in soils has aroused great interest in recent years. Designing and optimizing such an energy exploitation system usually requires a full understanding of the temperature distribution and evolution in the soil, which is usually realized by numerical models. In order to accurately and efficiently obtain the evolution characteristics of soil temperatures in the process of heat storage, the authors proposed a layered slice algorithm. Due to its superior solving performance, the proposed algorithm provides a useful tool for optimizing the design of buried-tube heat exchangers.

The research from Pokharel et al. [9] focused on laser heating and evaporation. They proposed a two-dimensional mathematical model for the laser heating and evaporation of a single droplet in the diffusion-dominated regime by taking into account the absorption of the laser radiation, evaporation process, and vapor flow dynamics. With the proposed model, the heat and mass transfer dynamics within the droplet during continuous and pulsed laser heating were explored. This model provides an additional tool for controlling heating and evaporation processes using a laser pulse.

Lyu et al. [10] studied waste heat recovery by focusing on an organic Rankine cycle (ORC). In this research, the authors developed a dual-loop ORC system to recover the waste heat from a marine main engine, in which exhaust gas heat was recovered by a high-temperature (HT) loop and the jacket cooling waste heat and the condensation heat of the HT loop were recovered by a low-temperature (LT) loop. The influence of different combinations of working fluids and working parameters on the ORC performance were studied and optimized.

García-Aranda et al. [11] examined the heat and mass transfer in a liquid/solid fluidized bed. In this research, the authors proposed a novel helical flow channel geometry for a liquid/solid fluidized bed heat exchanger, which allows for higher heat transfer rates and reduced complexity by operating below the particle transport fluid velocity. They optimized the combined fluidization parameters to obtain heat transfer coefficients and pressure drops and markedly improved heat transfer in the system.

The research from Ruan et al. [12] was concerned with the heat and mass transfer performance of an explosion puffing system for drying. Explosion puffing technology at low temperatures and high pressures is a type of compound-drying technology that enables the puffed product to obtain beneficial properties such as honeycombing, good rehydration, short processing time, etc. In this research, the authors established a mathematical model for the two-stage heating process to study the effects of the operation temperature, pressure, and time on the drying performance using fruits.

Fu et al. [13] studied the effects of a composite electrode structure—which plays an important role—on the optimization and performance of intermediate-temperature solid oxide electrolysis cells (IT-SOECs). In this research, the authors developed a three-dimensional multi-scale model for an IT-SOEC unit with a composite electrode, in which the electrode structure was modelled on a mesoscopic scale. With this model, they investigated the effects of operating pressure, volume fraction of the electrode phase, and particle diameter in the composite electrode on the electrolysis reaction rate, overpotential, convection/diffusion flux, and hydrogen mole fraction. The analysis of the IT-SOEC with composite electrodes using this multi-scale model enables the subsequent optimization of cell performance and composite electrode structure.

Janta-Lipińska et al. [14] proposed a method for the clean utilization of wastewater from mazout-fired boiler plants. Mazout is an alternative fuel for municipal energy boiler plants, which is much cheaper than other fuel oils. However, using mazout creates a problem with wastewater disposal, since the wastewater contains mazout and cannot be discharged into municipal sewage systems or natural water reservoirs. To solve this problem, the authors developed a system for the disposal of sewage containing mazout by specially preparing a water–mazout emulsion and then burning it in boilers. This system would not only reduce the amount of discharged sewage to zero but also allows the full utilization of the heat from the combustion of organic substances contained in the sewage.

Cao et al. [15] focused on the heat transfer within mechanical vapor recompression crystallization (MVRC) systems, which have been widely applied to treat salty effluents in an energy-efficient manner and can recycle a large amount of salt from the effluent. The horizontal drop film evaporator is a key component of the mechanical vapor recompression crystallization system, which can be used to evaporate the effluent and recycle secondary vapor energy. However, the heat and mass transfer characteristics of the salty effluent sprayed on the outer surface of the horizontal drop film evaporation tube are obviously different from that of ordinary water. In this research, the authors built a 3D model of the sprayed drop film with salty effluents on a horizontal tube heat exchanger, and they studied the heat transfer characteristics of the salty effluent flowing outside the tube in an MVRC system. The research results could provide a reference for the design of horizontal drop film evaporators in MVRC systems.

The research from Shen et al. [16] outlines a phase difference measurement method for Coriolis mass flow meters (CMFs). According to the measurement principle of CMFs, the mass flow rate is calculated by measuring the phase difference or time interval between two signals detected by electromagnetic sensors. To solve the poor precision problem in phase difference measurements with unknown frequencies in engineering practice, the authors proposed a new phase difference measurement method based on correlation theories, which showed good performance with respect to phase difference estimations and effectively improved the measurement accuracy of CMFs.

In addition to the research papers mentioned above, this Special Issue also contains a review paper highlighting the mathematical models for the flow and heat transfer of microcapsule phase change slurries (MPCSs). MPCSs are widely used in different fields such as thermal regulation or heat storage. Accurate numerical modelling is very important for studying the heat and mass transfer of MPCSs in energy systems. This review summarized the mathematical models proposed for the thermal processes of MPCSs, compared the adaptabilities of different models, and provided recommendations for further studies that will develop the models so as to enable more widespread applications of MPCSs.

The topics of the selected papers in this Special Issue cover various areas of heat and mass transfer research either at the macroscopic scale or microscale by using experimental or numerical models and studying fundamental principles or applications. Altogether, these studies represent, to some extent, the active research interest in this field.