Jorge Rojas

The lattice Boltzmann equation for natural convection in a two-dimensional cavity with a partially heated wall

The lattice Boltzmann equation method in two dimensions was used to analyse natural convective flows. The method was validated with experiments in an open cavity with one of the vertical walls divided into two parts, the lower part conductive, the upper part and all the other walls adiabatic. An upward thermal boundary layer formed near the conductive wall. This layer gave way to a wall plume. The numerical results compared well with experiments in the laminar (Ra =2 .0 × 10 9 ) and transition (Ra =4 .9 × 10 9 ) regimes. The behaviour of the starting plume was numerically studied for Rayleigh numbers Ra from 10 6 to 4.9 × 10 9 . The wall plume grows in three stages: in the first with constant acceleration, in the second with constant ascending velocity and in the third with negative acceleration due to the presence of the top boundary layer. The acceleration of the first stage and the velocity of the second both scale with the Rayleigh number.

Natural convection of a high Prandtl number fluid in a cavity

Transient natural convection in a square cavity heated with a time-dependent heat flux on one vertical wall and cooled by maintaining the opposite wall at a constant temperature was studied experimentally and numerically. The working fluid was silicon oil (Dow Corning fluid 20–200) with a Prandtl number of 230. All experiments were carried out in a cubic cavity of 0.13 m in each side. The heating rate used was 460 W/m2, which corresponds to a Rayleigh number of 4×109. Experimental data included temperature records at particular points and velocity measurements obtained from video images of tracers. The dynamics of the transition to the steady state is characterized by a vortex structure that forms near the heated wall. This structure is generated by shear at the heated wall boundary layer. The results were compared with a numerical simulation and qualitative agreement was obtained.

Transient natural convection in a cavity with heat input and a constant temperature wall on opposite sides

Abstract The transient natural convection of a fluid with Prandtl number of order 200 in a two-dimensional square cavity has been numerically studied. One of the vertical walls of the cavity is kept at a constant (ambient) temperature and a constant heat flux is applied on the opposite wall. The other walls are adiabatic. Initially, a boundary layer is formed near the heated wall; subsequently, a large vortical structure is generated, together with an upper intrusion layer. As time progresses, the average temperature in the cavity increases, and a descending boundary layer is formed near the constant temperature wall. During the transition to the steady-state regime, a thermal stratification pattern is formed. The results are compared with the scale analysis presented by Patterson and Imberger (1980).

Evaluation of refrigerator/freezer gaskets thermal loads

The gasket total thermal load is calculated as the heat transfer load plus the infiltration load. A method is developed to evaluate the gasket heat transfer load due to convective, radiative, and conductive mechanisms. It uses a quasi-one-dimensional theoretical model combined with experimental and numerical results. The gasket infiltration load is evaluated using the tracer gas method and humidity measurements. The gasket total thermal load obtained by these methods agrees with the load estimated by the reverse heat leak method, with the advantages of having a smaller uncertainty and providing information about the heat transfer along the perimeter of the gasket.

Thermal performance of two envelope systems: Measurements in non air-conditioned outdoor test cells and simulations

Measurements obtained from two non-air-conditioned outdoor full-scale test cells during a year in Torreón, Coahuila, Mexico, are used to compare the thermal performance of two envelope constructive systems for walls and roofs of monolithic concrete buildings. The two constructive systems have the same thickness, one is mono-layered and other is two-layered. The two-layered constructive system has a better thermal performance due to its larger thermal resistances and thermal capacity, and the more-layers effect. The experimental data were compared with numerical simulation results obtained with EnergyPlus using the time-dependent heat transfer model. The numerical results are, in general, in good agreement with the experiments. The surface lag time is the variable with the largest differences.

Transition on a Partially Heated Vertical Wall

Transition from laminar to turbulent flow, on a partially heated vertical wall (an adiabatic wall on top of an isothermal wall), was numerically and experimentally analyzed. The main objective was to find the location of the beginning and end of the transition based on the frequency spectra of the temperature signals obtained with a two-dimensional numerical model. The beginning of thermal transition was defined as the position where a higher frequency appeared (first harmonic) superimposed on the single laminar frequency. Further up, during transition, two or more harmonics were distinguished. The end of transition was defined as the location where no more dominant frequencies were found. A finite-volume procedure was implemented for the numerical solution. A schlieren system, thermocouples, and particle image velocimetry were used in the experiments to validate the numerical model. Four cases were considered with temperature differences of 1.1, 2.2, 4.4, and 6.6 K. The beginning and end of thermal transition in the partially heated boundary condition occurred at similar locations as in the isothermal wall, although temperatures along the adiabatic wall were considerably lower than the corresponding values in the isothermal wall.

DEVELOPMENT OF A WALL PLUME FROM A BOUNDARY LAYER ALONG A PARTIALLY HEATED VERTICAL WALL

Transient natural convection flow along a partially heated vertical wall was studied experimentally, using schlieren flow visualization, temperature measurements and particle image velocimetry. Departing from a motionless and in thermal equilibrium state, motion was generated by increasing the temperature of the lower half wall while the top half was thermally insulated. Four main events were registered: 1) Firstly, in the lower half wall, the boundary layer thickened and the flow started to ascend. 2) A group of waves traveled along the boundary layer. 3) From heated wall/adiabatic wall point, a starting plume was formed and a vortex was generated which moved towards the free surface. 4) Behind the vortex, a wall plume was formed leading to a steady state. Temperature difference between the heated wall and the fluid was varied in order to achieve conditions with three different Rayleigh numbers of 2.0 × 10 9 , 4.9 × 10 9 and 8.9 × 10 9 . The nondimensional velocity of the ascending vortex was described in terms of the product Ra 1/2 Pr -1/2

Numerical and experimental study of transient natural convection in an inclined wall cavity

Transient natural convection in an open cavity with one inclined wall is analyzed both numerically and experimentally. The fluid and the cavity are in thermal equilibrium at the onset of the experiment. The inclined wall is heated in such a way that the wall temperature increases uniformly according to an hyperbolic tangent function. The transport equations are solved using a 2-D transient model with a non-orthogonal body fitted coordinate system and an exponential grid distribution for better spatial resolution near the inclined wall. Measurements of velocity and temperature are performed at some key points of the boundary layer and intrusion layer. The fluid motion and heat transfer are analyzed from the time at which heat is applied through the inclined wall to the time at which its effect is detected at the opposite sidewall. The main patterns of the fluid flow and heat transfer are well predicted as comparisons against experimental results indicate.

Recursive Scheme for Sequential Leaks’ Identification

This chapter deals with the problem of detection and identification of multi-leaks in a single, horizontal pipeline assuming that only flow and pressure sensors at the ends of the pipeline are available. A characteristic of this monitoring scenario with simultaneous leaks is that the events are undistinguishable in steady state. This means the leaks could only be identified during transient behaviors. A monitoring problem, close to the simultaneous leaks’ issue, is the leaks’ scenario appearing in sequence. It is shown here that the isolation task is feasible with this scenario in the framework of model-based methods. Thus, a general recursive scheme which is formatted with three coupled nonlinear input–output equivalent models in steady state is proposed. Since the scheme is based on the equivalence model condition between one and multiple leaks’, previous to the presentation of the scheme, the static relation between equivalent models for one and multiple leaks is derived by considering the friction as a function of the flows. The interrelated three input-output models have the property to retain the data of the past leaks’, which allows an on-line identification of the new event during a time window for any arbitrary number of leaks. In particular the identifiers are implemented by using extended Kalman filters. The algorithm is tested with synthetic data simulated with Pipeline Studio software for a sequential set of three leaks, and it shows successful results.

Thermal Plume and Stratification

Visualization by synthetic schlieren of the filling box process with fluid heated by a thermal source located on the floor. A constant power of 73 W is released in a 25 cm tall, closed, insulated box filled with water. The initial temperature of the fluid is uniform and equal to the environment.