Condensation on a spray of water drops: a cell model study-II. Transport quantities

The formulation for a unit cylinder cell model that was used to analyze the hydrodynamics and heat transfer associated with steam condensation on a spray of equal sized water droplets was presented in part I of this study. In this part II, we report the results and discussions for the condensation induced interfacial velocities, surface shear stress, Nusselt number and the Sherwood number. The heat transport in both phases and the species transport in the continuous phase have been treated as transient processes. The interactions between neighboring drops have been examined. Numerically obtained transport results have been compared with an experimental study. Results for a representative spray show that the use of correlations developed for an isolated drop to predict condensation spray behavior may be inaccurate, although isolated drop studies continue to merit investigations.     

A spherical cell model for multi‐component droplet combustion in a dilute spray

A model for sphericosymmetric thin‐flame combustion of a multi‐component fuel droplet in a dilute spray has been developed using a unit cell approach. The gas‐phase transport has been modelled as convective–diffusive while the liquid‐phase processes as transient–diffusive. Convective heat and mass transfer condition has been used at the cell surface. The results indicate that evaporation and combustion characteristics of the droplet are strongly affected by the variation of both ambient conditions and convective transfer coefficients. Using the model, the effects of droplet spacing in spray, ambient oxidizer concentration, ambient temperature and pressure have been considered. Droplet life increases with decrease in droplet spacing, ambient temperature and ambient oxidizer concentration. However, droplet life has a weak dependence on ambient pressure. Copyright © 2001 John Wiley & Sons, Ltd.     

An Investigation of Capillary Flow Effect on Condensation Heat Transfer on a Grooved Plate

A simple model predicting the effect of capillary flows on the condensation heat transfer occurring on a vertical grooved plate is presented. The model includes the effect of capillary flows along the axial direction of the grooves and the direction from the apex to the cornered region of the groove. Numerical results indicated that the capillary flow induced by the capillary force significantly affects the condensation heat transfer, as does the plate height, groove width, and groove angle. In order to verify the theoretical prediction, an experimental investigation was conducted, and the theoretical prediction agrees well with the experimental data. Results of the investigation will assist in optimizing the condensation heat transfer performance in heat pipes.     

流速对非等温凝结表面Marangoni凝结换热特性的影响

在具有温度梯度的凝结表面上进行了水-酒精混合蒸气的Marangoni凝结的实验。研究了3种流速(V=2、4和6 m/s)对凝结表面上不同位置热流密度的影响。研究发现:流速对纯水蒸气和混合蒸气凝结时的表面温差和热流密度的影响是不同的。对纯水蒸气而言,流速增大后,表面温差和热流密度是增加的,并加剧了热流密度的分布不均(热流密度的相对差值在压力为31.2 kPa,流速2 m/s时为0.538,流速4 m/s时为0.6,流速6 m/s时为0.625)。对于混合蒸气,表面温差随流速的增加而减小,而热流密度增大很少(压力31.2 kPa,流速2 m/s时为0.186,4 m/s时为0.182,6m/s时为0.098)。     

Analysis of the spray field development on a vertical surface during water spray-quenching using a flat spray nozzle

The aims of this study were (i) to conduct experimental spatially and time-resolved measurements of flow development on large heated surfaces during transient spray cooling operations and (ii) to investigate and discuss the influence of spray cooling mechanisms such as bubble formation and the flow field development of the cooling fluid and how this affects heat transfer. Quartz plates were heated to above 500 °C and then sprayed with pressurised water subcooled to 80 K. High speed images of the quench process were collected at a rate of 3000 Hz making it possible to track the movement of the quench front as the plate cools below the Leidenfrost temperature of the fluid at that location. Observations showed that the relative importance of droplet-surface interactions decreases once the Leidenfrost temperature is reached on the plate: It was found that once the water contacts the surface, a water pool develops rapidly which grows larger as the pool floods the heated surface. Comparisons between the spatial flow development and heat transfer on the plate are made in order to describe these interactions more accurately. This information not only provides crucial input into process simulations, but is also useful to develop theoretical models of fluid–solid interaction describing the wetting of a heated component due to water spraying.     

Numerical simulation of a water spray—Radiation attenuation related to spray dynamics

Radiation attenuation by a water spray is predicted on the basis of a detailed simulation. First of all, a two-way coupling treatment of the spray dynamics is achieved through an Eulerian–Lagrangian modeling. Droplet distribution combined with water vapor and carbon dioxide volume fractions are then used to compute the radiative properties of the medium. A simulation of radiation propagation is performed, aimed at the computation of the spectral transmittance through the spray. A Monte Carlo technique is used to describe radiation absorption and scattering phenomena for a real droplet polydispersion and an equivalent monodispersion. Numerical results of spray attenuation are compared to experimental data obtained on a laboratory spray with low flow rate. Satisfying accuracy can be obtained for the numerical prediction if a realistic size distribution is used for the pulverization. The mean Sauter diameter and the volumetric fraction of droplets are found to vary with the position in the spray. Tentative predictions with a monodispersion therefore fail in predicting the attenuation ability of the spray at various vertical positions below the injection point.     

湿蒸汽非均质高速凝结流动的数值研究

基于冠状成核机理,建立了湿蒸汽两相非均质凝结流动数值模型,对缩放喷管、汽轮机叶栅和汽轮机级内湿蒸汽两相非均质凝结流动进行了数值模拟．结果表明：与自发凝结相比,非均质凝结流动中杂质颗粒改变了凝结过程;杂质颗粒减小了喷管中凝结激波强度,改变了汽轮机叶栅中的压力分布,降低了蒸汽过冷度,减少了不平衡热力学损失;在汽轮机级内,非均质凝结流动的动、静叶进、出口汽流角接近过热蒸汽流动的动、静叶进、出口汽流角,其动叶前压力高于过热蒸汽的动叶前压力,但级反动度偏离过热蒸汽流动数值．     

Coupling of a continuum fuel cell model with a discrete liquid water percolation model

An iterative algorithm is developed to directly integrate a discrete liquid water percolation model into a 3D continuum fuel cell model. In the continuum model the thermodynamic processes, most relevant for the water management and fuel cell performance, are calculated. For the discrete liquid water distribution in the porous transport layer (PTL), a water path network model is used, calculating the discrete, injection pressure and condensation scenario dependent saturation distribution.     

Effects of spray characteristics on critical heat flux in subcooled water spray cooling

Effects of spray parameters (mean droplet size, droplet flux, and droplet velocity) on critical heat flux (CHF) were studied while these parameters were systematically varied. The effect of each parameter was studied while keeping the other two nearly constant. The mean droplet velocity (V) had the most dominant effect on CHF and the heat transfer coefficient at CHF (h_c), followed by the mean droplet flux (N). The Sauter mean diameter (d₃₂) did not appear to have an effect on CHF. By increasing V, CHF and h_c were increased. This trend was observed when all other spray parameters were kept within narrow ranges and even when relaxed to wider ranges, indicating the dominant effect of V. The effect of N, although not so much as V, was also found to be significant. Increasing N resulted in an increase in CHF and h_c when other parameters are kept in narrow ranges. A dilute spray with large droplet velocities appears to be more effective in increasing CHF than a denser spray with lower velocities for a given N. The mass flow rate was not a controlling parameter of CHF.     

Evaporative heat transfer characteristics of a water spray on micro-structured silicon surfaces

Experiments were performed to evaluate the evaporative heat transfer characteristics of spray cooling of water on plain and micro-structured silicon surfaces at very low spray mass fluxes. The textured surface is made of an array of square micro-studs. It was found that the Bond number of the microstructures is the primary factor responsible for the heat transfer enhancement of evaporative spray cooling on micro-structured silicon surface in the present study. A qualitative study of evaporation of a single water droplet on plain and textured silicon surface shows that the capillary force within the microstructures is effective in spreading the deposited liquid film, thus increasing the evaporation rates. Four distinct heat transfer regimes, which are the flooded, thin film, partial dryout, and dryout regimes, were identified for evaporative spray cooling on micro-structured silicon surfaces. The microstructures provided better cooling performance in the thin film and partial dryout regime and higher liquid film breakup heat flux, because more water was retained on the heat transfer surface due to the capillary force. Heat transfer coefficient and temperature stability deteriorated greatly once the liquid film breakup occurred. The liquid film breakup heat flux increases with the Bond number. Effects of surface material, system orientation and spray mass flux were also addressed in this study.     

Non-adiabatic capillary tube flow: a homogeneous model and process description

This paper presents a homogeneous model of refrigerant flow through capillary tube–suction line heat exchangers, which are widely used in small vapour compression refrigeration systems. The homogeneous model is based on fundamental conservation equations of mass, momentum and energy. These equations are solved simultaneously through iterative process. Churchill’s correlation [3] is used to calculate single-phase friction factors and Lin et al. [6] correlation for two-phase friction factors. The single-phase heat transfer coefficient is calculated by Gnielinski’s equation [5] while two-phase flow heat transfer coefficient is assumed to be infinite. The model is validated with previous experimental and analytical results. The present model can be used in either design calculation (calculate the capillary tube length for given refrigerant mass flow rate) or simulation calculation (calculate the refrigerant mass flow rate for given capillary tube length). The simulation model is used to understand the refrigerant flow behaviour inside the non-adiabatic capillary tubes.     

水平光管内R410A与R134a流动冷凝换热特性的对比研究

为研究R410A与R134a在水平光管内的冷凝换热特性,在管内冷凝换热试验台上进行冷凝试验,分析质量流量、冷凝温度、测试水雷诺数Re、管径和制冷剂物性对换热系数和压降的影响。研究表明:换热系数、压降均随着质量流量的增加而变大,随冷凝温度的升高而减小,换热系数随测试水雷诺数Re的增加而减小,而测试水雷诺数Re对压降的影响相对较小;尽管R410A的换热系数随管径的减小而增大,而管径对R134a换热系数的影响并不显著,R134a与R410A的压降均随管径的减小而增大;单位压降换热系数随质量流量的增加而减小; Cavallini et al.关联式可较好预测R410A与R134a在光管内换热系数,而Shah关联式只能用于预测R134a的换热系数。     

Multi-Objective Optimization of R-404A Vapor Condensation in Swirling Flow Using Genetic Algorithms

Applying twisted tape inserts as a passive improvement technique increases both pressure drop and heat transfer coefficient. In the design of heat exchangers, decreasing of pressure drop and increasing of heat transfer coefficient simultaneously comprise an important aim. In this study, multi-objective optimization is used to find optimum combinations of heat transfer coefficient and pressure drop during condensation of R404A vapor inside twisted-tape-inserted tubes. At first, Pareto-based multi-objective optimization is used to find the proper artificial neural networks based on the experimental data for prediction of heat transfer coefficient and pressure drop. In the next step, Pareto-based multi-objective optimization and previously obtained artificial neural networks are used to find optimal operation conditions that lead to optimum combinations of heat transfer coefficient and pressure drop. The corresponding optimal set of design variables, namely, mass velocity, vapor quality, and dimensional parameters of tubes, show the important design aspects.     

A model of photon-induced self-driven electrochemical cell for water splitting to hydrogen

An equation for cell current in a self-driven photon-induced electrochemical cell, having both electrodes as semiconducting photoelectrodes, has been derived and applied to water splitting to hydrogen. The cell current and the cell potential depend on various semiconductor properties and the properties of the ions in solution. The computed dependence of cell current and potential for specific combinations of electrodes, e.g. nSrTiO₃/p-GaP and nTiO₂/p-GaP, show the same trends as the experimental observation. Further calculations suggest that it should be possible to attain an efficiency of conversion of light up to 18% for water splitting to hydrogen with p-InP (Pt-electrocatalyst)/n-Si (electrocatalyst) and up to 17% with p-Si (Pt)/n-InP(c) using appropriate electrocatalyst on the nSi and on the nInP electrodes.     

A two-dimensional steady state model including the effect of liquid water for a PEM fuel cell cathode

A two-dimensional steady state model for a PEM fuel cell cathode is described in this work. All the components in the cathode such as the gas manifold, diffusion layer, microporous layer and the catalyst layer are modeled. The effect of the liquid water is taken into account in every layer of the cathode. The model was developed and simulated using a combination of Maple and MATLAB. The combination provides a flexible framework for quickly developing models with various assumptions and different complexities. The cathode catalyst layer was modeled using both macrohomogeneous and spherical agglomerate characterizations. The model is validated using experimental data. During model validation, various assumptions are considered for transport within the porous layers of the cathode. Subsequently, the assumptions and characteristics that best predicts the experimental data are highlighted. The major conclusion of this work is that a model that includes liquid water in all the layers with a flooded spherical agglomerate characterization for the reaction layer best predicts the PEM fuel cell behavior in terms of an i–v characterization for a wide range of reactant flow rates. The utility of the steady state model for the optimization of the cathode catalyst layer design parameters is also described.     

A New Numerical Approach for Predicting the Two-Phase Flow of Refrigerants during Evaporation and Condensation

This article reports on four finite-volume–based numerical methods developed for predicting the one-dimensional two-phase flow of pure refrigerants in evaporators and condensers during change-of-phase processes. The methods differ in the physical assumptions considered at the interface separating the liquid and vapor phases and in the equation used to predict the variation of the refrigerant flow quality during change of phase. In all methods, numerical predictions are obtained via a locally iterative marching-type solution algorithm. Therefore, the models permit the prediction of the size of the pipe needed to achieve full evaporation/condensation of the saturated refrigerant. The effectiveness and robustness of the numerical procedures in predicting the flow and heat transfer characteristics are assessed by comparing results with published experimental data. Good agreement is obtained. The new approach is used to perform a parametric study analyzing the effect of refrigerant type, pipe diameter, and mass flow rate on the flow and heat transfer characteristics in evaporators.     

水滴在流动油介质中的碰撞特性研究

根据新型制冰方式,采用数值模拟方法对水滴在流动油介质中的传热、碰撞接触及合并特性进行了研究。对油水多相流动混合物中的连续介质油和离散水滴分别建立适时动态分布参数模型和颗粒轨道模型,模型建立过程中考虑到流体的适时物性参数、所喷入水滴粒径的变化,通过跟踪水滴轨迹,研究和获得了水滴在低温油介质中由水冻结成冰颗粒过程中所发生的碰撞和运动规律,为探讨新型制冰方法的可能性及实验研究打下了基础。