Parallel Vertical Channels Research Articles

Functional principles of the envelope characterized by the variable thermal resistance of the wall as a result of parietodynamic exhaust effect (PDE)

The paper presents the opaque envelope system in the endowment of inhabited buildings characterized by the use of the heat of the air discharged from the inhabited spaces, circulated through the space adjacent to the living environment / outdoor environment in order to reduce the need for heat / sensitive cold of the inhabited spaces (PDE). It is presented mathematical modeling of the processes of heat transfer and flow of indoor / outdoor air circulated through parallel vertical channels. The dimensions of vertical channel opening and air flows are based on the solutions of the property transfer equations associated with Model 1 (forced laminar flow) and Model 2 (globally representative model of both laminar flow and turbulent flow). Model 2 includes both laminar flow (space heating in cold season and cooling spaces in summer season hours with sun) and turbulent flow (cooling living spaces in summer season, hours of night – free cooling). A case study and the interpretation of the results obtained regarding the energy performance of the living space in relation to similar results associated with the conventional envelope is presented.

Application of cooling processes for the impact of variable thermal conductivity on the MHD flow of kerosene-based nanofluid

This paper studies the recent flow phenomena based upon the kerosene-based nanofluid through vertical parallel channel for the consideration of the variable thermal conductivity. The free convection of electrically conducting MHD fluid due to the interaction of inertial drag along with thermal radiation, heat source and the dissipative heat enriches the study as well. The structural behavior of the physical quantities is useful in recent applications like the cooling processes in refrigerators, semi-cryogenic rocket engine, etc. because of the practical application of the kerosene-based nanofluid. Also, the models associated with the thermophysical properties such as viscosity and thermal conductivity for the choice of volume concentration are favorable for the thermal enhancement. The designed model for the transport phenomena is transformed to its non-dimensional form for the suitable choice of similarity variables and the set of equations are handled by traditional numerical technique. Further, the involvement of the physical parameters and their physical significance is described briefly for the appropriate values within the specified range. Finally, the important outcomes of the study are that nanoparticle concentration has vital role in decelerating the fluid velocity and the fact is due to the heavier density and the inclusion of porous matrix is useful to retard the wall thickness at both the ends. Moreover, the increasing concentration favors in enhancing shear rate at the first wall whereas near the second wall, the impact is opposite.

Time‐dependent natural convection fluid flow of stably stratified fluid in an asymmetrically heated vertical channel filled with an anisotropic porous material

AbstractThis paper presents a theoretical analysis of the combined effects of anisotropic porous material and thermal stratification on the transient natural convection fluid flow in an asymmetrically heated vertical parallel channel. The solutions of the governing equations for the temperature and velocity fields are obtained using Laplace transform technique, Riemann sum approximation, and the D'Alembert method. The choice of the D'Alembert method is to provide a simple decoupling procedure for the coupled governing equations while still retaining their original orders. The research established that owing to the layering effect induced by the thermal stratification , the temperature and the velocity distributions of the fluid are found to be attenuated with an increase in thermal stratification. It is also observed that the inclusion of anisotropic parameters in the transport equations aids in regulating the fluid velocity, temperature, Nusselt number, skin friction, and mass flow rate. In addition, by neglecting the anisotropic parameter and taking into account the adiabatic stratification of the fluid, the numerical values for the mass flow rate of the present research favorably compared with the numerical results obtained by Singh et al.

Impact of Thermal Stratification on Unsteady Natural Convection Couette Flow Formation in a Vertical Channel Filled with Anisotropic Porous Material

Recently, heat transfer problems where anisotropic porous medium or stably stratified fluid are taken into account have been separately studied. Developing a mathematical model that combines these physical quantities naturally results to complex coupled differential equations. In this paper, a fully developed time dependent natural convection Couette flow of stably stratified fluid between vertical parallel channels filled with anisotropic porous material is investigated. The governing partial differential equations are transformed into ordinary differential equations using Laplace transform techniques and then decoupled using D’Alembert method. Exact solutions in Laplace domain for the velocity and temperature equations are then obtained. A numerical method: Riemann-sum approximation is then used to invert the expressions for the velocity and temperature profiles, as well as the resulting skin friction, rate of heat transfer and volumetric mass flow rate into their corresponding time domain. The research establishes that both the anisotropic and the stratification parameters aid in regulating the fluid temperature and velocity. The research further reveals that the fluid velocity attains its maximum (or minimum) velocity when θ = 900 (or θ = 00) for k*<1 and when k*>1, the fluid velocity is least (or maximum) when θ = 900 (or θ = 00).

WITHDRAWN: Analytical study on hollow and concrete in-filled columns with triangular and rectangular flutes under axial load

Abstract The ubiquitous stone column used in building construction in the ancient world was often fluted, it had parallel vertical grooves or channels cut into its surface. In this research, forty-two columns with different types of flutes and thickness were tested and their behaviour was studied. A Finite Element Analysis (FEA) model was developed to perform analysis of fluted columns subjected to compression. The cases with a different number of flutes such as 3, 4, and 5 respectively and with varying thickness of steel sheets 0.6 mm, 0.8 mm, 1 mm 1.2 mm, 1.4 mm, 1.6 mm, and 1.8 mm have been considered in this study. From the analytical investigation results it is understood that the increased number of flutes in columns directly effects on the load-carrying capacity of columns with five numbers of flutes having the highest load-carrying capacity, the lowest stress levels and less deflection relative to columns containing lesser number of flutes. The application of various thicknesses of column often has a strong impact on the ability to bear the load. The column with D/t of 88.88 exhibits greater load capacity, stress reduction, and lower deformation values compared to rest of column with different thickness.

Experiments and analyses of supercritical CO2 flow instability with study of wall heat-storage and dimensionless parameters

This paper reports both experiments and analyses of supercritical CO2 in two heated vertical parallel channels. The intent of the present experiments was to search for static instability in an up-flow system. However, no static instability was found using CO2. A total of 18 experiments at different flow conditions were performed with supercritical CO2 flowing upwards. Eleven were already published by Saini et al. [1] and 7 new cases are presented here. For all cases, oscillatory flow instability was observed. The 18 flow instability cases were numerically modeled to assess the performance of the in-house developed linear, frequency-domain program without and with wall-heat storage. The predicted numerical results agreed well with the experimental results in all cases. The study showed that including wall-heat storage, for this two channel systems, has a very minor effect on the flow instability boundary. Additionally, the accuracy of the supercritical flow dimensionless parameters proposed by Ambrosini and Sharabi [2], to convert experimental CO2 instability data to water within the same loop, was examined. The dimensionless parameters performed very well. There was also very good agreement between experiment and code on the oscillation period.

Experimental Investigation of Flow Instability Between Two Vertical Parallel Channels Using Supercritical CO2

Abstract Supercritical flow experiments were conducted at University of Manitoba using supercritical flow facility-vertical (SFF-V), which is a two vertical parallel channel assembly. The working fluid was CO2 at supercritical pressure and was driven by natural convective forces rather than a pump. Different system pressures (7.4 MPa–9.1 MPa), inlet temperatures (7 °C–30.1 °C) and various outlet-channel k-factors were used. A total of eleven parallel channel out-of-phase instability boundary points were obtained and the details of those cases are reported herein. These results can be used for code validation, to enrich the limited database of supercritical parallel-channel instability and to lend further insight into supercritical flow instability in heated parallel channels.

Numerical study on the effect of dissymmetry heating on flow instability of supercritical water in two parallel channels

The effect of dissymmetry heating on flow instability of supercritical water in two parallel vertical upward channels is investigated by a time-domain model. It is found that dissymmetry heating imposes different effects on the system stability under different inlet mass flow or inlet fluid temperature. This is because that mass flow distribution presents different response characteristics to heat load distribution, and critical mass velocity (Gcr) determines this characteristics. When average mass velocity in the channel (G) is below Gcr under low inlet mass flow, the system presents positive response characteristics and system stability increases with dissymmetry heating degree. With the increase of inlet mass flow, positive response characteristics turn to negative response characteristics when G is higher than Gcr. When the system presents negative response characteristics, system stability decreases with dissymmetry heating degree. The stronger the system response characteristics, the more obvious the effect of dissymmetry heating on system stability.

Heat transfer of viscous fluid in a vertical channel sandwiched between nanofluid porous zones

Mixed convection in vertical parallel channels is analyzed with the viscous fluid sandwiched between nanofluids within porous material filled in a vertical channel. The concept of single-phase transport of nanofluids is employed to define the nanofluid flow and heat transfer and the Darcy approach is incorporated to describe the circulation within the porous material. Formulated ordinary differential equations which are non-linear and coupled along with the corresponding boundary and interface conditions are solved by the regular perturbation method. The main objective is to investigate the effects of the Grashof and Brinkman numbers, solid volume fraction, porous parameter on the velocity and temperature fields. Results are shown in the graphical and tabular form. The physical characteristics governing the flow such as skin friction and rate of heat transfer are also investigated considering five different materials of nanoparticles.

Mixed Convection on MHD Flow with Thermal Radiation, Chemical Reaction and Viscous Dissipation Embedded in a Porous Medium

In this paper, a theoretical analysis has been made to study the effect of mixed convection MHD oscillatory Couette flow in a vertical parallel channel walls embedded in a porous medium in the presence of thermal radiation, chemical reaction and viscous dissipation. The channel walls are subjected to a constant suction velocity and free stream velocity is oscillating with time. The channel walls are embedded vertically in a porous medium. A magnetic field of uniform strength is applied normal to the vertical channel walls. The nonlinear and coupled partial differential equations are solved using multi parameter perturbation techniques. The effects of physical parameters, viz., the radiation absorption parameter, Prandtl number, Eckert number, dynamic viscosity, kinematic viscosity, permeability of porous medium, suction velocity, Schmidt number and chemical reaction parameter on flow variables viz., temperature, concentration and velocity profile have been studied. MATLAB code is used to analyze theoretical facts. The important results show that an increment in the radiation absorption parameter and permeability of porous medium results in an increment of the temperature profile. Moreover, an increment in the Prandtl number, Eckert number and dynamic viscosity results in a decrement of the temperature profile. An increment in suction velocity results in a decrement of the velocity profile. An increment in the Schmidt number, chemical reaction parameter and kinematic viscosity results in a decrement of the concentration profile.

Numerical Study of Heat and Mass Transfer during the Evaporative Drying of Porous Media

The paper deals with numerical study of drying process of porous media of sand during the evaporation of a liquid saturated porous layer within parallel vertical channel. The liquid and air streams are modeled as two coupled laminar boundary layers incorporating non-Darcian models of the inertia and boundary effects. The governing equations and the associated boundary conditions are discretized by means of the finite volume method implemented on a staggered mesh and the velocity-pressure coupling is processed by the SIMPLER algorithm. The influences of the inlet mass flow of the drying gas, porous layer thickness and the porosity on the drying process are analyzed. Results show that the drying rate of the porous media is improved by the reduction of the porosity and porous layer thickness a large drying rate is obtained with high inlet mass flow and high inlet gas temperature.

Enhancing the phase distribution in parallel vertical channels with single and double chamber coaxial headers

In this paper, in order to maximize the degree of flow rate uniformity of the two phase flow distribution into parallel vertical channels, a number of distributor fittings have been tested with respect to a test section characterized by a horizontal header and a series of vertical upward channels. The fluids are air and water and the flow regimes at the header cover the intermittent and annular flow pattern conditions. The ranges of liquid and gas superficial velocities are 0.45–1.2 and 1.5–16.5 m/s, respectively. A header geometry without any inserts, such as the smooth reference header, is taken as a reference case. Data from a number of inserts to enhance the uniformity of distribution are compared to the reference case, such as a series of cylindrical distributors with in-line holes along their length (“flutes”). New data in particular are presented for the “double chamber flute” fitting. As in previous studies by the Authors, extracted mass flow rates of liquid and gas from parallel channels are recast in terms of normalized flow rates and normalized standard deviation of phases. A new overall performance parameter is used for data reduction and for summarizing the comparison between the flute fitting configurations.

On the drag effect of one fluid driven by another in a vertical channel

The idea of dragging a viscous fluid by another kind of fluid via the shear stress has fascinated the scientists and engineers. The dependence of the drag effect on the physical parameters of the two immiscible fluids is very much desired but still challenging. In this research, three different kinds of fluids are employed to drag a pure fluid between vertical parallel channel walls, that is, the viscous fluid, the non-Newtonian power-law fluid, and the nanofluid. The drag effects of two-layer fluids are investigated by comparing the velocity fields and the mean velocity curve. Essential parameters determining the dragging efficiencies of the driven fluid are studied systematically: the drag effects of the density ratio p, the thermal conductivity ratio k, the thermal expansion coefficient ratio b, and the viscosity ratio m of the two-layer fluids are focused. Both dilatant flows and pseudo-plastic fluids are considered in driving the viscous fluid. When the pure fluid is driven by the nanofluid, the single-phase model is adopted. The example of 47 nm-Al2O3 nanoparticles suspended in water is analyzed for demonstration: the thermal expansion, the effective viscosity, and the effective thermal conductivity are dependent of the concentration of nanofluid, which makes the nanoparticle volume fraction ϕ a major concern in the drag effects. The findings in the paper reveal several potential strategies to promise high effectiveness on fluid driving via interface shear, which we hope will inspire engineers and researchers in relative working fields.

Mixed convection heat transfer of double immiscible fluids in functional gradient material preparation

Double immiscible flow has been extensively emerged in science and technology. In this research, mixed convection heat transfer of double immiscible fluids in a vertical parallel channel is investigated. One layer is pure fluid while the other is nanofluid. The effective kinematic viscosity and the effective thermal conductivity are regarded as linear functions of the nanoparticle loading parameter. A method of translation and scaling the computational domain for calculating the coupled equations subject to eight boundary conditions is developed. This method is applicable no matter the widths of the two regions are equal or not. Two interesting phenomena are revealed: the nanofluid with lower particle loading parameter is a better option for higher effectiveness of fluid driving; adding more nanoparticles in one layer can enhance heat transfer of both layers.

Phase split in parallel vertical channels in presence of a variable depth protrusion header

The air and water flow distribution are experimentally studied in a test section simulating a heat exchanger composed by a round header and 16 parallel upward channels. The effects of the tube protrusion depth as well as the gas and liquid superficial velocities are investigated and the results are compared with previous data. A new fitting solution to be inserted in the header has been developed based on previous findings by the Authors. The fitting geometry belongs to the family of the protruding pipes but the protruding depth has been varied along the header in order to cope with expected liquid and gas mass flow rates in the parallel channels. The flow at the header inlet is intermittent and annular and water and air have been employed as two phase mixture. The new header fitting demonstrated to yield meaningful improvements in phase distribution in terms of either dimensionless liquid and gas flow ratios or standard deviation of phase flow ratio (the overall performance parameter STD2 here presented decreased by 64%) when compared to the previous configurations.

CFD Simulations Devoted to the Study of Fitting Effects on the Phase Distribution in Parallel Vertical Channels

Abstract Uneven distribution of phases in plate heat exchangers is a cause of reduction in both thermal and fluid-dynamic performances. With respect to two-phase flows, phase separation in manifolds with several outlets is a complex phenomenon and no general rules are available for predicting the phase distribution at header–channel junctions. The design of compact heat exchangers and their distributors is still based on empirical approaches and both experimentation and numerical analyses are needed for defining the best geometries able to reduce the mass flow rate non-uniformities in parallel channels. In this paper, a series of CFD simulations are carried out to infer the effects of a protrusion fitting (inside the header) on the single-phase distribution in parallel upward vertical channels fed by a common horizontal distributor. The numerical results are compared with both experimental single-phase and two-phase (liquid/gas) experimental data. The effects of the operating conditions are investigated and general conclusions on the differences and analogies between single-phase and two-phase flows in the present problem are discussed.

Experimental study on the performance of a novel structure for two-phase flow distribution in parallel vertical channels

Uniform flow distribution is critical to obtain high thermal performance in many heat and mass transfer devices. It especially plays an important role in a compact heat exchanger. In this paper, a two-phase flow distributor is proposed for the evaporator unit of the plate-fin heat exchanger to alleviate the phase maldistribution in the multiphase flow. Air and water mixture was adopted as two-phase medium and distributions into ten parallel channels were measured in detail. The results show that the proposed distributor can improve the two-phase flow distribution of the plate-fin heat exchanger.

Augmentation of heat transfer in a bubble agitated vertical rectangular channel

This paper presents the results of an experimental study of convective heat transfer between three parallel vertical plates symmetrically spaced with and without bubble agitation to ascertain the degree of augmentation of the heat transfer coefficients due to agitation. The centre plate was electrically heated, while the other side plates were water-cooled forming two successive parallel vertical rectangular channels of dimensions 20 cm × 3.5 cm × 35 cm (length W, gap L, height H) each. At the bottom of the hot and cold plates air spargers were fitted. Water/ethylene glycol (100%) was used to fill the channels. The superficial gas velocity ranged from 0.0016 to 0.01 m/s. Top, bottom and sides of the channels were open to the water/ethylene glycol in the chamber which is the novel aspect of this study. Experimental data have been correlated as under: Natural convective heat transfer: Nu = 0.60 Gr0.29, r = 0.96, σ = 0.186, 1.17 E6 < Gr < 1.48 E7; Bubble agitated heat transfer: St = 0.11(ReFrPr2)−0.23, r = 0.82, σ = 0.002, 1.20 E−2 < (ReFrPr2) < 1.36 E2.

The effect of the flow direction inside the header on two-phase flow distribution in parallel vertical channels

Abstract Uniform fluid distribution is essential for efficient operation of chemical-processing equipment such as contactors, reactors, mixers, burners and in most refrigeration equipment, where two phases are acting together. To obtain optimum distribution, proper consideration must be given to flow behaviour in the distributor, flow conditions upstream and downstream of the distributor, and the distribution requirements (fluid or phase) of the equipment. Even though the principles of single phase distribution have been well developed for more than three decades, they are frequently not taken in the right account by equipment designers when a mixture is present, and a significant fraction of process equipment consequently suffers from maldistribution. The experimental investigation presented in this paper is aimed at understanding the main mechanisms which drive the flow distribution inside a two-phase horizontal header in order to design improved distributors and to optimise the flow distribution inside compact heat exchanger. Experimentation was devoted to establish the influence of the inlet conditions and of the channel/distributor geometry on the phase/mass distribution into parallel vertical channels. The study is carried out with air–water mixtures and it is based on the measurement of component flow rates in individual channels and on pressure drops across the distributor. The effects of the operating conditions, the header geometry and the inlet port nozzle were investigated in the ranges of liquid and gas superficial velocities of 0.2–1.2 and 1.5–16.5 m/s, respectively. In order to control the main flow direction inside the header, different fitting devices were tested; the insertion of a co-axial, multi-hole distributor inside the header has confirmed the possibility of greatly improving the liquid and gas flow distribution by the proper selection of position, diameter and number of the flow openings between the supplying distributor and the system of parallel channels connected to the header.

Radiative effects on natural convection in vertical convergent channels

Natural convection in air, in a convergent channel, uniformly heated at the principal walls, is experimentally investigated, in order to analyze the effects of the radiative heat transfer. Results in terms of wall temperature profiles as a function of the walls inclination angle, the spacing between the walls, the heat flux, are given for two values of the wall emissivity. Flow visualization is carried out to show the peculiar pattern of the flow between the plates in several configurations. The comparison between two wall emissivity values, 0.10 and 0.90, shows that the effect of thermal radiation is more pronounced for larger convergence angles. For a wall emissivity equal to 0.90 and for small values of the minimum channel spacing, heat transfer in slightly convergent vertical channels is stronger than in a vertical parallel channel. Flow visualization points out a recirculating zone in the upper part of the channel for small values of the minimum channel spacing and for converging angles equal to 5° and 10°. Nusselt numbers and dimensionless maximum temperatures are then evaluated and correlated to the Rayleigh number, in the investigated range from 5 to 5 × 10 8 and 0° ⩽ θ ⩽ 10°. A very good agreement between experimental data and correlations is observed for the dimensionless parameters based on the maximum channel spacing. Comparisons between experimental and numerical data are also performed and a good relationship is observed.