Conjugate Natural Convection with Surface Radiation from Different Infrared Suppression (IRS) Systems

Abstract

The naval ships, warships, and merchant ships are equipped with infrared suppression (IRS) systems for easy operation in the deep ocean. The IRS device reduces the temperature of hot exhaust products, which will help in stealth technology so that infrared gadgets cannot detect ships or aircraft. The cooling of the IRS system is an important aspect, and the current research deals with the numerical investigation of various IRS systems, namely, cylindrical funnel, IRS with multiple cylindrical and conical funnels, louvered cylindrical funnel, louvered conical funnel, and conical funnel. This research primarily aims to obtain thermo-fluid characteristics from the mentioned IRS systems due to conjugate heat transfer with the combined effect of natural convection and surface radiation. In addition, the time required to cool down the hot IRS system to atmospheric temperature is also estimated using the lumped-capacitance method. For the numerical analysis, different governing equations (Navier-Stokes equation, turbulence equation (𝑘−𝜖 equation), energy equation, and radiation equation) are solved using the finite volume method (FVM) based solver of ANSYS Fluent 15.0. Initially, the preliminary work is carried out on the vertical hollow cylinder with a finite thickness of different aspect ratios to observe the combined effect of convection and surface radiation. Results show that including surface radiation in the net heat transfer rate estimation is significant and should not be ignored. The research is further extended for the IRS system with multiple cylindrical and conical funnels. To perform the analysis, different parameters such as the number of funnels, Rayleigh number, inner surface temperature, geometric ratio (GR), funnel overlapping, and surface emissivity are varied to elucidate the heat transfer behavior. The results of the IRS system with cylindrical funnels show that the total heat transfer rate rises with the number of funnels. Both non-dimensional induced mass flow rate and total heat transfer rate have maximum value for zero overlapping cases irrespective of funnel numbers. In contrast, it decreases for both negative and positive overlapping. At constant temperature contribution of radiative heat transfer varies from 10 to 38% with the rise in emissivity. In the case of a conical funneled IRS system, an increase in geometric ratio enhances the mass suction and total heat transfer rate with funnel numbers for the considered temperature range. At a constant temperature, the total heat transfer increases up to 4.9 times when GR varies from 1 to 1.3 for six funneled systems. Convective heat transfer’s relative strength rises with Rayleigh number, whereas the relative strength of radiative heat transfer drops. However, radiative heat transfer contributes up to 63% of total heat transfer for fixed GR and funnels. The convective heat transfer is always higher than radiative heat transfer for IRS with multiple funnels; therefore, radiative heat transfer contribution weakens with increased funnels for all considered emissivities, whereas the convective heat transfer contribution augments. Furthermore, the thermodynamic characterization of IRS with multiple cylindrical funnels subjected to natural convection alone is carried out. The Bejan number decreases with the Rayleigh number and geometric ratio. As the number of funnels rises, the increment in heat transfer irreversibility is much lesser than the fluid friction irreversibility, resulting in a continuous decrease in the Bejan number. In the case of a cylindrical louvered funnel, some geometrical parameters like the number of holes in each row (6 to 14), number of rows of louvers (2 to 8), and shapes of holes are also varied in addition to Rayleigh number, surface temperature, and surface emissivity. The results indicate that the Nusselt number and mass suction ratio increases with the number of rows of holes and the number of holes in each row. The Nusselt number is highest for the funnel with circular holes, whereas the lowest is for triangular holes. Moreover, results show that the surface radiation contribution is significant even at low emissivity. Furthermore, the research is extended towards the three-dimensional comparative study of the IRS system made up of a single cylindrical louvered funnel, conical louvered funnel, cylindrical funnel, and conical funnel. The mass suction ratio and total Nusselt number increase with the wall temperature of the surface linearly. The conical funnel gives the least total Nusselt number, and the cylindrical louvered funnel gives the highest. Moreover, thermodynamic analysis of these funnels is also conducted, and results reveal that the cylindrical louvered funnel outperforms the rest of the considered funnel in terms of heat transfer rate, increasing with surface temperature and Rayleigh number. A performance evaluation criterion (ratio of irreversibility to heat transfer) is described for the performance of different funnels, and results indicated that it decreases with Rayleigh number, indicating that the rise in heat transfer becomes much more than the rise in irreversibility at high Rayleigh. Also, the time-temperature graph shows that the louvered cylindrical funnel cools the fastest while the louvered conical funnel is the slowest. To visualize the thermo-fluid behavior around the IRS system and its near surrounding various temperature contours, velocity vector plots, velocity contours, and entropy generation contours for different cases are also provided for better understanding.