FLUID FLOW AND HEAT TRANSFER IN VENTILATED ENCLOSURES AND DESIGN OF PRECAST CLEAN ROOM

Abstract

With the advent of increasingly efficient insulation materials, cooling/heating loads and thereby air supply rates have significantly reduced in the management of indoor environment. As a result, modeling of laminar and turbulent flows has gained increasing importance. In mechanically ventilated and air-conditioning system inlet and exhaust positions play key roles in the cooling process and influence both energy consumption and cooling efficiency. In mixed convection, the interaction between forced and natural convection plays an important role in controlling the rate of heat transfer, and temperature distribution in an enclosure. Mixed convection in ventilated enclosures with one isolated heat source located one of the vertical wall in which the interaction between an internal buoyancy flow and an external forced flow determines the fluid flow and heat transfer structures has received considerable attention in the last decade. Many researchers have computationally investigated the effect of room geometry, and different inlet/outlet arrangements on mixed convection air-cooling of buildings [61, 73, 78, 96, 98, 118, 121, 131]. Double-diffusive mixed convection has wide applications in engineering as for example in cooling of electronic circuit boards, nuclear reactors, solar energy storage and environmental control of buildings, etc. [34, 91, 92, 113, 159]. The review of the earlier research indicates that only few investigations [42, 119, 145] have studied the problem of mixed convection subjected to discrete contaminant source along the bottom surface and heat source along the right vertical wall or bottom wall. However, a detailed study is required to understand the double diffusive mixed convection with two discrete heat and contaminant sources on the ventilated enclosures. It is evident from previous studies [134, 145, 147, 148, 150] the ventilated rooms with i ii inlet air is injected through the inclined jet, the cooling performance was improved. However, in order to comprehensively understand the effect of inlet port inclination on indoor air environment, further investigations are needed to build upon the leads provided by earlier studies. Such an investigation is presented in this thesis. The mathematical tools presented in this thesis for analyzing air-flow in enclosures have practical applications in the design of clean rooms which find wide applications in electronics manufacturing, food processing and controlled environment facilities in hospitals etc. A proposal for the design and construction of modular hard-wall clean rooms which have potential use as sanitised relief shelters in war and in disaster-prone zones has been presented in the thesis. In order to achieve economies of scale and encourage mechanisation of construction, the use of precast concrete for the construction of the aforesaid clean rooms has been discussed in detail. Towards encouraging practical application, all illustrative solved example on the structural analysis and design of a precast concrete modular hard-wall clean room has been presented in the thesis. Outline of the thesis The present thesis is divided into six chapters and the chapter wise description is given below. Chapter 1 presents an introduction to the thesis. A brief introduction to partial differential equations governing flow behaviour and the development of Computational Fluid Dynamics (CFD) is also included in this chapter. Particular attention has been paid to discussion of Navier-Stokes solver in CFD which is a powerful tool for handling the nonlinear partial differential equations in fluid flow. In Chapter 2, fluid flow and heat transfer in a mechanically ventilated enclosure using mixed air-distribution system at various levels have been numerically studied. Thermally activated fluid is supplied through various slots of the heated left vertical wall and flushed out through a port in the opposite wall. The horizontal walls are assumed to be impermeable and adiabatic. Six different placement configurations of the inlet and outlet ports are studied. iii Steady, laminar, and incompressible flow under Boussinesq approximation have been considered. The governing equations including continuity, momentum and energy are solved by the finite volume technique based on Semi-Implicit Method for Pressure-Linked Equations (SIMPLE) algorithm. The cooling efficiencies are found to be maximum when the inlet and the outlet are placed at the middle and lower parts of the hot and the cold walls. In Chapter 3, airflow, heat and contaminant transfer in a mechanically ventilated twodimensional rectangular enclosure under a laminar flow regime have been numerically simulated. Two different inlet-outlet port enclosure configurations have been considered. In configuration-A, the cold air is injected at the top of the left vertical wall, and exited at the bottom of the right vertical wall. In configuration-B, the cold air is injected at the lower edge of the left vertical wall, and exited at the top of the right vertical wall. In both the configurations two discrete heat and contaminant sources are placed on the bottom and right vertical wall, respectively. The objective of the study is to find the relative locations of inlet and outlet in order to obtain more effective cooling in the core of the enclosure by maximizing the heat and contaminant removal rate and reducing the overall temperature. The governing equations in Cartesian co-ordinates are solved by finite volume based SIMPLE algorithm. Results are presented for different values of the Reynolds number, Grashof number, Sherwood number and buoyancy ratio in the laminar regime. A convective transport visualization technique is used to study the behaviour of physical phenomena due to stream function, heat function and mass function. The results indicate that cooling inside the enclosure is most effective when the inlet is kept at the bottom of the left vertical wall. In Chapter 4, a numerical study of mixed convective cooling in a two-dimensional inclined jet enclosure under a laminar flow regime is carried out. An external fluid enters the enclosure through an inlet port on the left vertical wall and exits through the right vertical wall, the horizontal walls are assumed to be adiabatic and the vertical walls are maintained at different temperatures. Four different placement configurations of the inlet and outlet ports are studied. The governing equations including continuity, momentum and energy are solved by the finite volume technique based SIMPLE algorithm. The angle of the inlet fluid iv is varied in the range of 0o−30o in order to gain new insights into the formation of different flow patterns of streamlines and isotherm lines. The objective of the investigation report in this chapter is to identify the optimum placement of inlet and exit ports for best cooling effectiveness. For all the configurations, it is observed that cooling efficiency improves with increase in the angle of inlet air. A detailed analysis of the results are presented in the form of streamlines, isotherms, and average Nusselt number with varying in Richardson number and Reynolds number. It is found that in the configuration with the inlet port at the bottom of the left vertical wall and the outlet port at the top of the right vertical wall, the natural and the forced convection assist each other in the heat removal process. In Chapter 5, a proposal for modular clean roomsmade using precast concrete have been presented and discussed. One of the practical applications of controlled environment is in clean rooms which are widely used in specialised manufacturing industries and hospitals etc. Such clean rooms are expected to find applications under emergency situations which may be encountered during war and natural or man–made calamities. Structural configuration and behaviour of precast concrete clean rooms have been discussed together with materials, analysis and design strategies and the options for detailing of such structures have been presented. It have been proposed that precast inverted U-frames are an attractive option for the construction ofmodular hard-wall clean rooms. It is shown that by suitably stacking these modular units next to each other clean rooms with a variety of plain shapes and dimensions can be easily assembled at site. This advantage together with the other properties of concrete in terms of its neat and clean formed surfaces are shown to justify the proposed use of precast concrete for the construction of portable clean rooms. As an aid to practical application, a solved example and its analysis and design of an inverted U-frame modular clean room have been presented in the thesis. Finally, chapter 6 presents the conclusions drawn from the thesis and scope for further investigations.