查看更多>>摘要：Through the internal cooling from evaporation of tiny water droplets during the compression process, wet compression can effectively augment the net power output of gas turbine under high ambient temperature condition. In this paper, effects of wet compression on compressor performance were theoretically and experimentally investigated on one of stages of a multistage centrifugal compressor and a new performance correction model for wet compression was proposed. This model is based on corrected similarity criterion and couples with equivalent gas properties of wet compression, droplet evaporation model and droplet dynamics loss. The results show that wet compression can increase pressure ratio and efficiency and reduce specific compression work at same pressure ratio. The effect enhances with the increase of water injection ratio and the decrease of droplet average diameter. The essence of influence on performance is the variation of equivalent isentropic exponent. For this stage, maximum pressure ratio and peak efficiency increase by 4.25% and 0.71% respectively with a water injection ratio of 2% and a droplet average diameter of 10 mu m.
查看更多>>摘要：This work aims to analyze the heat and mass exchange of complex bubbling. First, an empirical correlation for estimating bubbling heat transfer coefficient in different superficial air velocities is obtained based on previous works. Then, a modified theoretical model is set up and used to discuss the performance of the bubbling HDH system. Results show that higher input power enhances water productivity and thermal efficiency, but a maximum exists. When input power is constant, the influence of air flow rate on water productivity is remarkable when air velocity is not large. More importantly, the system has optimal cooling water flow rate and temperature. The optimal cooling water flow rate increases with input power, whereas the optimal cooling water temperature decreases with input power. When input power is 1000 W, the optimal cooling water flow rate and temperature are 60 L/h and 59 degrees C, respectively. In addition, ambient temperature variations show a very small effect on system productivity and thermal efficiency. The results of this paper can provide theoretical reference for the design of a bubbling HDH desalination system.
查看更多>>摘要：Thermal management plays an increasingly important role for internal combustion engines due to the high demands on the transient response of vehicles and the legal requirements for pollutants. In particular, the goal to reduce exhaust-gas raw emissions while optimizing fuel consumption can be supported with the help of on-demand temperature control. One strategy to meet the thermal requirements connected to this consists of model-based approaches. For an implementation of such methods on an automotive electronic control unit, a sufficiently high accuracy has to be ensured by the underlying model, alongside the fulfillment of the real-time operation demand. In this publication, we suggest two analytical modeling approaches for heat exchangers with poor thermal isolation with respect to the surrounding. Here we lay a special focus on the real-time capability of the given algorithm for an automotive on-board application. To this end, we utilize the Hammerstein method which allows to divide the overall physical system into a nonlinear stationary part and a dynamical linear one. The former are based on the well-known concept for the dimensionless temperature change of heat exchangers, where we generalize this approach in order to take account of the heat-losses to the surrounding. We demonstrate our model for an indirect charge-air cooler in an internal combustion engine. For both models we find excellent accuracy, with an overall mean-absolute error of 1.23 K and 1.33 K respectively, when compared to experimental data sets containing measurements from the Worldwide harmonized Light Duty Test Procedure.
查看更多>>摘要：The transformation to a truly sustainable energy system will require taking better advantage of the waste heat. Integrating heat exchangers with the triply periodic minimal surface (TPMS) is a promising and efficient way to build waste heat recovery systems that harness heat emissions from the low pitch thermal systems. This is mainly due to the low hydrodynamic resistance and pressure drop in the TPMS while securing good heat transfer at low-temperature gradient. This study establishes a computational design and analysis of heat and mass transfer inside a heat exchanger based on the TPMS structure and determine thermal effectiveness, heat transfer coefficient, and pressure drop inside the channel. The non-linearity dependence of results to several design variables makes obtaining the optimal design configuration solely using conventional CFD or experimental study nearly impossible. Hence, a multi-objective optimization workflow based on a Genetic Algorithm for laminar flow is employed to reveal the underlying relationships between design variables for the optimal configurations. The results illustrate the local sensitivity of important parameters such as the heat transfer coefficient, Nusselt number, and thermal performance of the heat exchanger against various design variables. It is shown that the pressure drop is directly affected by gas inlet velocity, viscosity, and density, from high to low, respectively. The Pareto frontiers for the optimal thermal performance are extracted, and the correlation between design objectives is determined. This methodology provides a promising framework for heat exchangers' design analysis, including multi-objective goals and design constraints.
查看更多>>摘要：Deep borehole heat exchangers (BHEs) can be utilised effectively for geothermal (heat) extraction from up to a few kilometres below the subsurface using the conventional closed U-loop or coaxial configurations. These systems take advantage of the high enthalpy energy in the deeper ground without relying on fracture flow extraction/injection, hence avoiding the complexity and uncertainty of hydraulic fracturing to enhance the permeability of deep aquifers. Despite the recent theoretical advancement in deep BHE systems, recent studies mainly assume conductive heat transfer in the ground, neglecting the effects of groundwater and heat convection (natural or forced) on the system's thermal capacity. This simplified assumption could result in underestimating geothermal extraction efficiency given the extensive fluctuations in groundwater thermo-physical properties, particularly at high temperatures. Therefore, the ultimate goal of this study is to investigate the extent to which the thermal performance of deep coaxial borehole heat exchangers (CBHEs) is affected by natural convection in deep aquifers. This is achieved by developing a novel numerical approach for reliable quantification of thermal yield, accounting for conduction and the commonly overlooked thermo-induced convection in aquifers -using a tested 3D finite element heat and mass transport model. Considering groundwater parameters' temperature dependency, the resultant buoyancy in groundwater and the effects of gravity, parametric studies are undertaken for various ranges of permeability, carrier fluid velocity and CBHEs depth. The overall validity of the model is tested against published experimental data. It is concluded that higher carrier fluid inflow rates for a given CBHE depth can lead to up to 43% higher CBHEs thermal efficiency when the aquifer's permeability is high (k = 10(-10) m(2)). Furthermore, the results indicate that a higher aquifer permeability, hence a substantial thermo-induced heat exchange in the aquifer, has a more pronounced effect on CBHEs thermal yield than carrier fluid inflow rate in shorter CBHEs (63% vs 17% increase), which contradicts the behaviour of longer CBHEs in which the thermo-induced convective heat transfer in aquifers with higher permeability, i.e., k = 10(-10) m(2) and carrier fluid inflow rate could have a similar effect on the system's efficiency (45% vs 43% increase). In general, it is concluded that an accurate evaluation of how system parameters such as the inflow rate and CBHE's depth could contribute to CBHE's thermal yield requires a good understanding of the impacts of aquifer's characteristics, i.e., temperatures, permeability and depth on thermo-induced heat exchange in the aquifer, which is facilitated using the proposed numerical model.
查看更多>>摘要：Packed beds are used for solar power energy storage, in the chemical and process industries, and in nuclear reactors. Mining ventilation coolth is the application of interest where the energy storage capabilities of rock aggregate may provide an economical and environmentally friendly heat exchanger solution. Particle-resolved simulations using ANSYS Fluent were obtained from uniform and Rosin-Rammler distributions of spherical and polyhedra particles within the pore Reynolds number range Re-pore = 15-3000. This paper increases confidence in using discrete element methods to numerically generate a random packed bed and demonstrates that computational capability exists to resolve over 1000 non-spherical particles. Following the principle of dimensional homogeneity, all packed beds are expected to conform to drag coefficient C-D = 38.363/Re-pore(0.928) + 0.291 and Nusselt number Nu = 0.430Re(pore)(0.580) Pr-1/3 + 1.274, meaning that coefficient of performance can be estimated for any rock aggregate.
查看更多>>摘要：In this study, we conducted molecular dynamics simulations of the evaporation of locally heated argon liquid to construct the kinetic boundary condition (KBC) for the evaporation. The KBC denotes the boundary condition of the Boltzmann equation, and the mass, momentum, and heat fluxes through the interface can be determined by solving the Boltzmann equation with the KBC. From the results, we established a method for constructing the KBC for the evaporation of locally heated argon liquid. Furthermore, we found that the velocity distribution of the KBC immediately after liquid heating becomes anisotropic, which means that the normal and tangential temperatures composed of outgoing molecules from the liquid phase to the gas phase take different values when the liquid interface is heated momentarily. From the present study, we can elucidate the mechanism of the occurrence of net evaporation mass flux due to the locally heated liquid film, e.g., the spot heating by infrared radiation.
查看更多>>摘要：This paper proposes the use of ceramic polishing slag to make fillers for evaporative coolers. The experimental tests on the heat transfer performance of the ceramic foam packing are carried out by using the independent test bench, focusing on the effects of the packing's flow channel form, drench density, the inlet air volume and pressure drop on the heat, and the mass transfer performance of the packing. The experimental results show that the optimum drench density of the ceramic foam packing in the form of a folded runner and a linear runner is 10.0 and 6.7 kg/(m center dot h), respectively. The direct evaporative cooling efficiency of the ceramic foam packing in both runners tends to decrease with the decrease in the dry and wet bulb temperature difference of the inlet air and the increase of the air mass flow rate. Under the same experimental conditions, the direct evaporative cooling efficiency, optimum inlet air velocity, and pressure drop of the packing in the form of a folded flow channel are higher than those of the linear flow channel. The optimal inlet air velocities of the packing in the form of a folded runner under the conditions of Urumqi, Xi'an, and Guangzhou are 1.6, 1.4, and 1.0 m/s, respectively, and the direct evaporative cooling efficiency can reach 83.01%, 70.74%, and 66.41%. In comparison, the optimal inlet air velocity for all three conditions under the linear runner is 1.0 m/s, and the direct evaporative cooling efficiencies are 73.78%, 68.56%, and 63.28%, respectively.
查看更多>>摘要：The supercritical CO2 used as heat transfer fluid (HTF) in solar tower cavity receiver has higher pressure and operating temperature, and lower investment than traditional heat transfer fluid, such as water steam, molten salt, etc. Many researchers have analyzed the effects of mass flow rate, heat flux, tube diameter, operating pressure and inlet temperature near the critical point on the heat transfer performance of supercritical CO2. However, there are few studies on the convective heat transfer characteristics of supercritical CO2 in tower solar receivers under the non-uniform heat flux boundary. In this contribution, in order to improve the convective heat transfer performance of supercritical CO2 under non-uniform heat flux, we firstly analyze the effect of the arrangement of the receiver tube on the flow and heat transfer performance of supercritical CO2. Then, we apply the field synergy principle to guide the flow field enhancement, and design the non-uniform structure at the position with larger field synergy angle. Finally, we further analyze the effect of the non-uniform structure on the heat transfer performance of supercritical CO2. The results show that the triangular ribs tube is the best heat transfer enhancement structure, e.g., when the mass flow rate is 0.1 kg.s(-1), the Nusselt number is 956.11, which is 163% higher than that of the smooth tube. This contribution can provide a guidance for the structural enhancement of a practical supercritical CO2 solar receiver to improve the heat transfer performance.
查看更多>>摘要：The efficient cooling technology used in first-stage turbine vanes results in a large temperature difference between the blade wall and high-temperature gas. The resulting thermal radiation cannot be neglected. However, in experimental environments, it is impossible to accurately assess the effects of radiation. In this study, a novel, simplified version of an existing experimental method is proposed, to distinguish the radiation from the total heat flux; it uses blade wall-temperature measurements and ternary nonlinear regression. The results show that when the mainstream temperature is 1000-1300 K and the cold air temperature is 550-750 K (T-infinity/Tcold > 2.5), the radiative heat flux accounts for 15-20% of the total heat flux (when the gas composition is pure air). In real working conditions (T-infinity/Tcold asymptotic to 2.2), the radiative heat flux cooled without an air film accounts for 11.96-14.4% of the total heat flux. When the gas includes radiation-participating media (e.g., CO(2 )and H2O), the radiative heat flux accounts for 21.4-22.72% of the total heat flux. The effects of radiation cannot be neglected in studies considering the heat transfer of turbine blades. In addition, the radiation correction factor is defined to comprehensively account for the effects of radiation. Under real conditions (T & INFIN;/Tcold asymptotic to 2.2), the radiation correction factor is 1.275. Finally, the simplified approach is validated using numerical calculations.
NSTL
Elsevier