Sumanta Acharya

Natural convection in the annulus between concentric horizontal circular and square cylinders

Numerical solutions are presented for natural convection heat transfer between a heated horizontal cylinder placed concentrically inside a square enclosure. Three different aspect ratios (R/L 0.1, 0.2, and 0.3), and four different Rayleigh numbers (Ra = 10, 10, 10, and 10), are considered. The governing elliptic conservation equations are solved in a boundary-fitted coordinate system using a control volumebased numerical procedure. Results are displayed in the form of streamlines, isotherms, maximum stream function estimates, and local and average normalized Nusselt number values. At constant enclosure aspect ratio, the total heat transfer increases with increasing Rayleigh number. For constant Rayleigh number values, convection contribution to the total heat transfer decreases with increasing values of R/L. For convection-dominated flows, the average Nusselt number correlation is expressed as Nu = 0.92Ra(R/ L)*. Generated results are in good agreement with previously published experimental and numerical data.

NATURAL CONVECTION IN TRAPEZOIDAL CAVITIES WITH BAFFLES MOUNTED ON THE UPPER INCLINED SURFACES

A numerical investigation has been undertaken to study the effects of mounting baffles to the upper inclined planes of trapezoidal cavities (representing a building or attic space). Two thermal boundary conditions are considered: (a) the vertical and upper surfaces are heated while the lower surface is cooled (summerlike conditions); (b) the lower surface is heated while the other surfaces are cooled (winterlike conditions). For each boundary condition, computations are performed for two baffle heights and two baffle locations. Rayleigh number (Ra) values range from 103 to 5 x 107 for summerlike conditions and from 103 to 106 for winterlike conditions. For both boundary conditions, results obtained with air as the working fluid reveal a decrease in heat transfer in the presence of baffles. In winterlike conditions, convection starts to dominate at an Ra much lower than that in summerlike conditions. The decrease in heat transfer becomes increasingly more significant as the baffle gets closer to the heated vertical wall for the bottom-cooled situation and as the baffle gets closer to the symmetry line for the bottom-heated case. In general, this decrease in heat transfer is higher with taller baffles. Average Nusselt number (Nu) correlations for both boundary conditions are presented.

Natural convection in a trapezoidal enclosure with offset baffles

A numerical investigation has been made of natural convection heat transfer in a trapezoidal enclosure (representing attic spaces) with offset baffles. Two thermal boundary conditions representing summerlike conditions (upper surface heated) and winterlike conditions (upper surface cooled) and two baffle heights are studied. For each boundary condition and baffle height, two baffle positions are considered. In position I, the upper baffle is offset toward the heated vertical wall and the lower baffle is offset toward the symmetry plane of the enclosure, whereas in position II, the upper baffle is offset toward the symmetry plane and the lower baffle is offset toward the heated wall. Rayleigh number values range from 10 3 to 5 × 10 7 for summerlike conditions and from 10 3 to 10 6 for winterlike conditions. Predictions reveal a decrease in heat transfer in the presence of baffles. In winterlike conditions, convection starts to dominate at a Rayleigh number much lower than that in summerlike conditions. The maximum reduction in heat transfer is achieved with long baffles placed in position II for summerlike conditions and in position I for winterlike conditions. Average Nusselt number correlation for both boundary conditions are presented.

Modal Analysis of Countercurrent Shear Flows

Momentum driven countercurrent flow in confined geometry has been simulated using large eddy simulation. Instantaneous flow features are investigated and three instability mechanisms are identified as (i) Kelvin-Helmholtz, (ii) Jet Oscillation and (iii) Elliptic Instability. Modal analysis is performed using (i) Dynamic Decomposition Method (DMD) and (ii) Proper Orthogonal Decomposition (POD) technique on the 3 dimensional data in domain of interest for control parameter (U1-U2)/(U1+U2) = -1.6 (ms/mp = 0.36). DMD and POD modes are compared and dynamically significant flow structures are identified and related to instability mechanisms.