Computational Insights Into Air-Side Flow and Heat Transfer in Compact Heat Exchangers

The paper describes two- and three-dimensional computer simulations which are used to study fundamental flow and thermal phenomena in multilouvered fins used for air-side heat transfer enhancement in compact heat exchangers. Results pertaining to flow transition, thermal wake interference, and fintube junction effects are presented. It is shown that a Reynolds number based on flow path rather than louver pitch is more appropriate in defining the onset of transition, and characteristic frequencies in the louver bank scale better with a global length scale such as fin pitch than with louver pitch or thickness. With the aid of computer experiments, the effect of thermal wakes is quantified on the heat capacity of the fin as well as the heat transfer coefficient, and it is established that experiments which neglect accounting for thermal wakes can introduce large errors in the measurement of heat transfer coefficients. Further, it is shown that the geometry of the louver in the vicinity of the tube surface has a large effect on tube heat transfer and can have a substantial impact on the overall heat capacity.

Steady State Analysis of Heated Soil Vapor Extraction Process With Air Sparging

Heated Soil Vapor Extraction (HSVE), developed by Advanced Remedial Technology is a Soil remediation process that has gained significant attention during the past few years. HSVE along with Air sparging has been found to be an effective way of remediating soil of various pollutants including solvents, fuels and Para-nuclear aromatics. The combined system consists of a heater/boiler that pumps and circulates hot oil through heating wells, a blower that helps to suck the contaminants out through the extraction well, and air sparging wells that extend down to the saturated region in the soil. Both the heating wells and extraction wells are installed vertically in the saturated region in contaminated soil and is welded at the bottom and capped at the top. The heat source heats the soil and the heat is transported inside the soil by means of conduction and convection. This heating of soil results in vaporization of the gases, which are then absorbed by the extraction well. Soil vapor extraction cannot remove contaminants in the saturated zone of the soil that lies below the water table. In that case air sparging may be used. In air sparging system, air is pumped into the saturated zone to help flush the contaminants up into the unsaturated zone where the contaminants are removed by SVE well. In this analysis an attempt has been made to predict the behavior of different chemicals in the unsaturated and saturated regions of the soil. This analysis uses the species transport and discrete phase modeling to predict the behavior of different chemicals when it is heated and absorbed by the extraction well. Such an analysis will be helpful in predicting the parameters like the distance between the heating and extraction wells, the temperature to be maintained at the heating well and the time required for removing the contaminants from the soil.

Heat Transfer Increase by Flow Structures Modifications

This paper makes use of a new methodology for heat transfer increase through flow structures modifications. Intended to help railway designers in handling cooling issues, it is applied to improve the roof-mounted equipment design of a modern railway coach, namely the CORADIA TER 2N NG produced by the ALSTOM Transport company. The brake resistor, a key equipment in charge of dissipating the train kinetic energy as heat into the surrounding air during braking phases, has been particularly considered. To do so, a simple model including a heated obstacle inside a three-sided lead-driven cavity is used, and simple geometry variations are suggested. Their impact on heat transfer is then estimated through numerical simulations while experimental tests validate the results obtained.

Evaluation of General Correlations for Heat Transfer During Boiling of Saturated Liquids in Tubes and Annuli

Six of the most verified correlations for boiling heat transfer were compared to data for horizontal and vertical tubes and annuli. The correlations evaluated were: Chen (1966), Shah (1982), Gungor and Winterton (1986), Liu & Winterton (1991), Kandlikar (1990), and Steiner and Taborek (1992). The database used to evaluate these correlations included 29 fluids: water, refrigerants, cryogens, organic and inorganic chemicals. The data cover reduced pressures from 0.005 to 0.783, mass flux from 28 to 11071 kg/m2s, vapor quality from 0 to 0.95, and boiling number from 0.000026 to 0.00742. The correlations of Shah and Gungor & Winterton gave the best agreement with data with a mean deviation of about 17.5%, only a couple of data sets showing large deviations. The paper presents and discusses the results of this study. Included are tables giving the range of dimensional and non-dimensional parameters covered by each experimental study.

PIV Measurements Within the Waves of Wavy and Wavy-Annular Horizontal Two-Phase Flow

A novel technique of microscopic Particle Image Velocimetry (PIV) is presented for two-phase annular, wavy-annular and stratified flow. Seeding of opaque particles in a water/dye flow allows the acquisition of instantaneous film velocity data in the film cross-section at the center of the tube in the form of digital image pairs. An image processing algorithm is also described that allows numerical velocities to be distilled from particle images by commercial PIV software. The approach yields promising results for stratified and wavy-annular flows, however highly bubbly flows remain difficult to image and post-process. Initial data images are presented in raw and processed form.

Axial Development of Vertical Upward Bubbly Flow in a Minipipe

Accurate prediction of the interfacial area concentration is essential to successful development of the interfacial transfer terms in the two-fluid model. Mechanistic modeling of the interfacial area concentration entirely relies on accurate local flow measurements over extensive flow conditions and channel geometries. From this point of view, accurate measurements of flow parameters such as void fraction, interfacial area concentration, gas velocity, bubble Sauter mean diameter, and bubble number density were performed by the image processing method at five axial locations in vertical upward bubbly flows using a 1.02 mm-diameter pipe. The frictional pressure loss was also measured by a differential pressure cell. In the experiment, the superficial liquid velocity and the void fraction ranged from 1.02 m/s to 4.89 m/s and from 0.980% to 24.6%, respectively. The obtained data give near complete information on the time-averaged local hydrodynamic parameters of two-phase flow. These data can be used for the development of reliable constitutive relations which reflect the true transfer mechanisms in two-phase flow. As the first step to understand the flow characteristics in mini-channels, the applicability of the existing drift-flux model, interfacial area correlation, and frictional pressure correlation was examined by the data obtained in the mini-channel.

Condensation of Steam on Finned Surfaces With Addition of a Surfactant

A fundamental knowledge of the parameters affecting film condensation is essential for the design of two phase heat exchangers. The current study examines the effect of extended surfaces and surface energy modifications and their interaction for condensation of steam in quiescent and vapor flow conditions. The enhancement of heat transfer for vertical, flat surfaces and two finned surfaces were compared for Reynolds numbers ranging from approximately 10 to 50. The addition of a nonionic surfactant, alcohol alkoxylate, to the system was evaluated for the same surfaces and vapor field conditions. Vapor flow of 0.25 m/s enhanced the heat transfer approximately 40%, while 0.5 m/s vapor velocity produced almost 100% increase in heat transfer. The addition of surfactant to the system produced small enhancement in heat transfer except in the case of condensate hold-up between the fins. In this case, the addition of surfactant increase the heat transfer an additional 25%, likely because the vapor flow and change of surface energy were sufficient to largely eliminate the hold-up of condensate between the fins.

CFD Optimization Application on Airside Plate Fins of Condenser Coil of Gravity-Assisted Heat Pipe

The heat transfer and fluid flow characteristics of airside plate fins of the condenser coil of a gravity-assisted heat pipe are numerically predicted. Based on these CFD computations, the optimization of the airside structure of the heat pipe is carried out by studying the influence of different geometry design variables on a combined heat exchanger evaluation function K, which can be changed by an engineer according to his application experience. In this work, one hundred of DOE’s points are sampled across the design space of the heat pipe based on the Latin Hypercube method. The Least Squares Fitting method is used to fit these sampled points to the RSM of the design space. The Mixed Integer Optimization Algorithm is used to explore the maximum specially-defined K on the Response Surface; and the corresponding optimal size and shape of the heat pipe are finally obtained. The optimization procedure is performed automatically by employing the optimizer-Optimus, grid generator-Gambit, and CFD solver-Fluent.

Nucleation Kinetics for Boiling in Microstructures

Theoretical investigation is conducted to understand the bubble nucleation process in microstructures. The bubble evolution in microstructures is investigated for momentum conservation, and the evolution rate is deeply dependent on the structure. According to different dynamic characteristics in the region close to the critical radius, the nucleation process is divided into two stages. Based on the characteristics of these two stages, a nucleation kinetic equation is modified from classical theory and then is conducted to understand the special bubble nucleation process. The result concludes that the nucleation rate will be deduced if bubble evolution is restrained in microstructures.

A Three-Dimensional Numerical Modelling of Atmospheric Pool Boiling by the Coupled Map Lattice Method

In the present paper, the characteristic atmospheric pool boiling curve is qualitatively reproduced for water on a temperature controlled thin copper strip having comparable length and breadth by the coupled map lattice (CML) method using a three-dimensional boiling field model. The basic objective of the work is to improve the prediction of the critical heat flux (CHF) with respect to the 2D CML model of Ghoshdastidar et al. [10]. The work models saturated pool boiling of water at 1 bar on a large (much larger than the minimum wavelength of 2D Taylor waves) and thin horizontal copper strip. The pool height is 0.7 mm, indicating thin film boiling. In the present model, it is assumed that boiling is governed by (a) nucleation from cavities on a heated surface, (b) thermal diffusion, (c) bubble rising motion and associated convection, (d) phase change and (e) Taylor instability. The changes with respect to 2D model are primarily with respect to 3D modelling of thermal diffusion and 2D distribution of nucleating cavity sizes. The predicted CHF is 1.57 MW/m2 as compared to the actual value of 1.3 MW/m2 and 0.36 MW/m2 predicted by the 2D CML model of Ghoshdastidar et al. [10]. It can be said that for the first time a coupled map lattice method which is essentially qualitative in nature has been able to predict the CHF of saturated pool boiling of water at 1 bar very close to the actual value.