查看更多>>摘要：This study investigates cold thermal energy storage (CTES) using a helical coil heat exchanger modified with bubble injection. One of the effective methods for increasing the heat transfer rate in heat exchangers is using bubble injection. A helical coil heat exchanger is immersed inside a cylindrical water storage tank, where the helical coil is the evaporator of a vapor compression refrigeration cycle (VCRC) and provides the designed cooling. Experimental studies were carried out to examine the impact of bubble injection on Nusselt number, the temperature differences in the storage tank, exergy degradation in the evaporator, and cycle coefficient of performance (COP). The bubbles were injected from the bottom of the storage tank in four different geometries at airflow rates ranging from 3 to 11 L/min. The experimental results of this study revealed that bubble injection could significantly increase the COP and heat transfer rate from the storage tank, as well as the exergy destruction and Nusselt number (Nu). This increase was highly dependent on the geometry and flow rate of the bubble injection. The results also indicated that the bubble injection has an optimal flow rate value, which was 9 L/min in this study. More specifically, the COP of the refrigeration cycle and the Nu number increased by 124% and 452%, respectively, compared to the non-bubble injection mode. Finally, for calculating the outside Nusselt number of the helical coil, an empirical correlation as a function of bubble flow rate and bubble injection angle was proposed.
查看更多>>摘要：Vapor quality and mass flux are crucial parameters for heat transfer coefficient and pressure drop of flow boiling. In this paper, they are actively adjusted by means of draining and refilling liquid refrigerant in the paths for evaporator performance enhancement. Principles of the vapor-liquid adjustment evaporator are introduced and its corresponding configuration is proposed. The mathematical model of this novel evaporator is established and validated by experiment data. Under different separation efficiencies, inlet vapor qualities and inlet mass flowrates, the vapor-liquid adjustment evaporator (AE) is investigated and compared to the conventional evaporator without vapor-liquid adjustment (CE) from the perspectives of overall performance and local behaviors. At the studied conditions, the improved heat transfer coefficient and reduced pressure drop are simultaneously obtained at a separation efficiency of 40%. Its superiority over the conventional evaporator is generally confirmed in terms of total temperature penalization. However, the excessive separation efficiency leads to the deteriorated performance due to the appearance of superheating. The major differences of local heat transfer coefficient and pressure drop between the vapor-liquid adjustment evaporator and conventional evaporator take place in the first and second paths, which are the consequences of the competition of the enhanced vapor quality and decreased mass flux. The stratified wave flow is mostly encountered for both evaporators. The vapor-liquid adjustment evaporator can be further enhanced by optimization. This study offers an innovative approach for flow boiling heat transfer enhancements.
查看更多>>摘要：To improve the utilization rate of geothermal resource using organic Rankine cycle (ORC), an organic Rankine flash cycle (ORFC) combined with insufficient evaporation and flash process is proposed to reduce the irreversibility of the power generation cycle. The thermodynamic performance and techno-economic performance of ORFC and two-stage series organic Rankine cycle (TSORC) are optimized, respectively. The effects of evaporation temperature, flash temperature and outlet dryness on the performance of two systems are discussed. The results show that with the increase of heat source temperature, the optimal operating condition value of the system increases, which leads to improving the thermodynamic and techno-economic performance significantly. R601a and R601 are the most suitable working fluids for TSORC and ORFC, respectively. The heat exchanger is the main component that causes the exergy destruction of the system, and its improvement potential is the highest in all components. The thermodynamic performance and techno-economic performances of ORFC are better than those of TSORC. The irreversible loss of the system can be reduced by the insufficient evaporation combined with flash separator.
查看更多>>摘要：As an effective method for heat management of hypersonic vehicles, regenerative cooling faces a severe problem of insufficient cooling capacity under high-speed conditions. Aiming to increase the cooling capacity of a given fuel, we conducted an optimization study by considering the influence of working conditions, chemical kinetics, and chemical routes. Via establishing a framework of multi-physical simulation by coupling the catalytic reactions with complex heat transfer process from subcritical to supercritical status, we conducted a parametric study of the effects of working conditions (i.e., inlet temperature and inlet velocity) to reveal the influence of physical heat sink, and different chemical kinetics and chemical routes to optimize the chemical heat sink. As a limiting case study, the surface coking process was also investigated. With the consideration of both physical and chemical heat sinks, the regenerative cooling capacity of a hydrocarbon fuel can be effectively increased via proper optimization. Using n-Decane as an example, a total heat sink of 2.5 MJ/kg is obtained under typical working conditions. A maximum heat sink of 5.3 MJ/kg could be obtained by engineering chemical routes with ethylene and hydrogen as the final cracking products, under inlet conditions of 473 K and 0.042 m/s. Results also reveal that it is essential to reduce the temperature of the wall to minimize carbon deposition. For practical applications, careful consideration of the synergies among the inlet conditions, reaction kinetics and routes, and coking should be performed to maximize the cooling capacity of a hydrocarbon fuel.
查看更多>>摘要：Low-temperature preheating to achieve effective thermal management for lithium-ion batteries is a crucial enabler for the efficient and safe operation of electric vehicles in cold conditions. Effective heating is yet challenging due to its implementation complexity and a tricky balance of the heating performance. Here, we develop a lightweight compound self-heating system involving two external light aluminum heaters, which recycle the discharge energy contributing to external heating. Basic electrical and thermal modeling for the compound selfheating system is performed and experimentally validated. We adopt four key but conflicting heating metrics: heating time, heating efficiency, battery degradation, and temperature uniformity, to optimize the resistance of external heaters with the adaptive particle swarm optimization method. We thus propose a rapid compound selfheating strategy that can conveniently warm the battery up with 32.49 degrees Cmin(-1). Experimental results under different states-of-charge and temperatures confirm the good adaptability of the proposed heating strategy. Comparison experiments with the unheated battery demonstrate the proposed heating strategy improves discharge power, charge power, and discharge energy by over 7.4 times, 19.0 times, and 109.9%, respectively. With the optimal external aluminum heaters, battery available discharge energy is enhanced by above 70.4%, implying a huge step forward to boost battery performance.
查看更多>>摘要：The organic Rankine cycle (ORC) is an efficient power generation technology that has been widely used in renewable energy utilization and industrial waste heat recovery. The thermal stability of working fluids has a significant effect on the fluid selection and system design of ORC systems. In this study, an off-design model of an ORC system was established with hexamethyldisiloxane (MM) as the working fluid, and the effects of the MM thermal stability on the system were analyzed. The results showed that the effect of MM thermal decomposition on the cold source and working fluid pump was limited. The outlet temperature of the evaporator decreased with MM decomposition, which might lead to incomplete evaporation of working fluids and possible damage to the expander. The outlet temperature of the heat source also decreased with MM decomposition, which led to lower outlet temperatures than the acid dew point limit temperature. Both of these results can affect the safe operation of ORC systems. The net power and thermal efficiency of the system decreased with increasing thermal decomposition ratios of MM. The net power and thermal efficiency decreased by 7.48% and 10.72% respectively in the model of this study with a 10% decomposition ratio.
查看更多>>摘要：Heat release and mass transfer characteristics in micro power unit with dual-fuel at high environmental load are mainly investigated in this paper. Energy conversion equations are solved in both phases separated by a regressing gas-solid interface. The igniter jet is assumed as a cross flow and then added to the gas governing equations in the form of source terms. Analyses have focused on variations in temperature, heat release rate per volume and species concentration. Results show that the forced-convection heat transfer in the gas flame zone is performed under the effect of the cross flow. Meanwhile, the rapid depressurization process of the micro system results in an adiabatic expansion phenomenon of the combustion-gas near the solid surface. However, the sandwich propellant can always keep burning due to the continuous addition of energy source from the igniter combustion-gas of high temperature. The out-of-phase blowout effect, that is, the gas zones with strong heat release always keep away from the solid surface, also appears in the whole process. Significantly, when the initial pressure or the average flow velocity of igniter jet is lower than 20 MPa or 40 m/s, the temperature distribution in the flowfield is greatly affected by the initial state.
查看更多>>摘要：Microstructured surfaces with spatial arrangements of hydrophilic and hydrophobic regions have been widely studied in boiling heat transfer enhancement. In this study, a conceptual design of microstructured surfaces with time-varying wettability is proposed to regulate bubble dynamics and thus enhance the heat transfer performance in the nucleate boiling regime. The effectiveness of the design is examined numerically for single-bubble heat transfer on a micropillar-arrayed silicon surface using a comprehensive three-dimensional model. The intrinsic contact angle is initially set as 48 degrees, it is subsequently reduced to 20 degrees in the bubble growth (0.152 and 0.4 ms) or departure stage (0.60 and 0.80 ms). The simulations show that two mechanisms contribute to heat transfer enhancement. First, the time-varying wettability decreases the adhesion force on the bubble exerted by the surface, thereby increasing the bubble departure frequency. Second, it also increases the local curvatures of the bubble, leading to the formation of thin liquid films between the bubble and micropillar walls. The liquid films suppress dry spots and hence enhance the heat removal of the bubble from the surface. The simulations also show that the reduction in the intrinsic contact angle enhances the evaporation on the liquid-vapor interface but deteriorates the evaporation in the microlayer. Because the former dominates the heat transfer process of the bubble on the micropillar-arrayed surface, the time-varying wettability increases the total evaporation rate. In the meantime, the time-varying wettability reduces the time proportion of the departure stage, which shortens the weakened heat transfer period in a bubble life-cycle and thus enhances the average heat transfer rate. As compared the case with constant wettability, the average heat transfer rate for the cases with time-varying wettability increases by 38.5%, 23.1%, 19.2%, and 15.4%, respectively, when the intrinsic contact angle is reduced at 0.152, 0.40, 0.60, and 0.80 ms, indicating that earlier wettability modulation gives rise to more significant heat transfer enhancement. The optimal time for wettability modulation is exactly the same as the time at which the total evaporation rate begins to decline.
查看更多>>摘要：The helical micro fin tubes (HFT) are commonly used in various double pipe heat exchangers because of the excellent processing and anti-fouling performance. It is of great significance to further improve the overall efficiency of the HFT so as to diminish energy consumption. In this work, the heat transfer and flow characteristics of the HFT are studied by numerical simulation. The results show that the heat transfer enhancement factors of the HFT are the secondary flow generated near the wall and the increase of the heat exchange area. In addition, the effects of the geometrical parameters on thermal-hydraulic performance are studied at Re = 36,636. It is found that the micro fin height (e), the helical angle (phi), and the number of starts (Ns) have a significant impact on the overall performance, and there is a strong mutual coupling between them. According to the parametric analysis, the HFT with a low micro fin height and a large number of starts is considered to be a better geometrical type. Finally, in order to select (or design) the HFT quickly under the specific working conditions, based on the exergy destruction minimization principle, the geometrical parameters are optimized by using the artificial neural network and genetic algorithm. An optimal solution (e = 0.23 mm, phi = 36.1 degrees, and Ns = 66) is selected from the Pareto front by the TOPSIS method. The results indicate that the optimal solution has a sensible balance between the exergy destruction caused by heat transfer and fluid flow. Besides, it has a better thermal-hydraulic performance as well (PEC = 1.73). This work fills the gap of heat transfer and the geometrical optimization study of HFT based on the second law of thermodynamics and provides strong evidence that the exergy destruction minimization principle is still applicable in the case of the periodic model and fully developed turbulence. We hope that it will be contributed to the structural design of the HFT.
查看更多>>摘要：In summer air-conditioning, conventional vapor compression systems cool the process air to its dew point temperature for dehumidification and further heat it to required supply temperature. This involves extra cooling and subsequent heating. Liquid desiccant systems avert this issue by separately handling latent and sensible loads, while running primarily on low grade thermal energy. Falling film type liquid desiccant dehumidifiers are advantageous over other types, mainly, as they reduce desiccant carryover into supply air stream. The current work aims towards the analysis and the performance improvement of a falling film type dehumidifier with the help of wavy profile of the working surface. Detailed comparison of the two commonly used desiccants, i.e., LiCl and CaCl2 are carried out to analyse the performance of the dehumidifier. The present work also includes the characterisation of the wave profile and the analysis in terms of various important parameters, such as, the dehumidification effectiveness, the moisture removal rate and the change in specific humidity, missing in earlier studies. In order to study the performance, a simplified 2-D transient finite volume model is taken into consideration to simulate a multiphase, multi-component conjugate heat and mass transfer system using ANSYS Fluent 19.2. The volume of fluid method is used to trace out the liquid-gas interface. With back-end modification of the solver, the thermophysical properties of the liquid film are evaluated using empirical relationships, as a function of temperature and concentration. The penetration model of mass transfer provides the local mass transfer coefficients. The moisture removing the potential of the aqueous solutions of LiCl and CaCl2 are eval-uated at quasi steady-state conditions. The numerical solver is validated with the already published experimental results with a maximum deviation of 10.31%. A sinusoidal wavy profile of the wall introduced in the system enhances the turbulence intensity by 3.8% and the dehumidification performance by 33.18% and 18% for LiCl and CaCl2, respectively. It is due to the enhanced liquid-gas interface area. The mean film thickness is found to improve by 15.25% and 13.38% for LiCl and CaCl2, respectively. The effect of process parameters on the per-formance of the dehumidifier is also studied. It has been observed that the dehumidification effectiveness de-creases from 86.14% to 31.1% with increase in inlet air velocity from 0.2 m/s to 2.0 m/s. It has also been found that a lower concentration of the desiccant solution and lower velocity of air yield higher dehumidification effectiveness. However, the moisture removal rate of the system will be compromised. Compared to the previ-ously studied cases, the current configuration of the sinusoidal profile with amplitude to wavelength ratio of 1:15, yields superior dehumidification performance.
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