NUMERICAL ANALYSIS OF HEAT TRANSFER FLOW IN VARIOUS TEXTURED TUBES AND SURFACES

Abstract

In today’s world where there is an active debate on global warming and energy crisis and to keep up with the energy demands; it remained a challenging task to produce energy efficient devices. Mostly, crucial effects on environment have been observed by the industrial sector which is a mega contributor in affecting the global warming. In this regard, efforts are made to improve the equipments, such that not only their efficiency gets improved but they also become less vulnerable to environment. Among many others, heat exchanger is also a commonly used device in industries. And it is not common to industries; it is used in daily used appliances as well, such as refrigerator, air conditioner, car radiator and computers. In the design of heat exchangers, where maximum research is focused on the optimization, most of the efforts are much inclined to improve the heat transfer and effectiveness. In this regard, surface augmentation has been actively researched in recent decades. Usually, this sort of enhancement is dominant on the tube side. It has been seen that the study is greatly conducted in the past experimentally, but numerical studies are limited to determine friction factor or Nusselt number. Only a few discussed an important factor called the entropy generation minimization. In this thesis, two kinds of analyses have been performed, 1) the analysis of groove tubes and 2) the analysis of textured surfaces (plates). The two topics are not linked ii directly to each other, but the essence of both works is same, i.e, to optimize the thermal enhancement factor. In the first part the study anchors on the application of optimization technique on the grooved tube design. The tubes contain internal grooves extruded along their axial length in a helical pattern. The thesis initially contains study of the literature review and then comparison of the numerical study using Computational Fluid Dynamics (CFD) with published experimental data was presented. The experimental data, examined tubes with helical grooves of different pitch length. In the thesis, after validation with experimental data, different pitch lengths other than experimental study were examined. After that, the work is based on optimization using Design of Experiment (DoE). Using this technique, the maximum thermal enhancement factor was determined as a function of number of starts, the groove-depth and the helical pitch length. It was found that the tube with maximum number of starts and the least pitch length and maximum groove-depth gives the best thermal enhancement factor. Finally, the entropy minimization study as a function of Reynolds number was conducted on the optimized tube. The optimum Reynolds number is at the point where the tube has observed minimum generated entropy with respect to the smooth tube. The second part is the numerical study of three shapes engraved into the surface of a plate and then thermal enhancement factor was studied due to the presence of these textures. The shapes were square, chevron and cylindrical. Computations were performed on these plates as well as flat plate geometry and the comparison was done by comparing thermal enhancement factor of the three groove designs. The Reynolds number were 10,000, 18,000, 26,000, and 28,000 with a constant surface heat flux of 12 kW/m2. The results of flat plate heat exchanger were validated by empirical relations. It was seen that Nusselt number and the heat transfer coefficient were highest in case of the cylindrical textured plates with 35% enhancement more than flat plate. Thus, it was concluded that heat transfer was maximum in case of the cylindrical textured plate. Keywords: Computational fluid dynamics, Design of experiment, Entropy generation minimization, Groove tube, Helical, Optimum, Plate, Reynolds number, Thermal enhancement factor, Turbulence.