Reduction in thermal stratification in two phase natural convection in rectangular tanks: CFD simulations and PIV measurements

• Published in: Chemical engineering science 2013-08, Vol.100, p.300-325
• Main Authors: Gandhi, Mayurkumar S., Joshi, Jyeshtharaj B., Nayak, Arun K., Vijayan, Pallippattu K.
• Format: Article
• Language: English
• Subjects: air; chemical engineering; Computational fluid dynamics; computer software; condensers; cooling; Draft tube; energy; fins; Fluid flow; heat exchangers; Heat transfer; heat transfer coefficient; industrial applications; Mixing; Particle Image Velocimetry (PIV); Pool boiling; Stratification; Tanks; temperature profiles; Thermal stratification; thermocouples; Tubes; Turbulence; Turbulent flow; wavelet
• ISSN: 0009-2509; 1873-4405
• DOI: 10.1016/j.ces.2013.02.064

Heat transfer by natural convection in a cavity is encountered in various industrial applications, such as heating and cooling of living spaces, fire in building, solar thermal collector systems, electronic and photovoltaic cooling devices, thermosiphon heat exchangers, passive decay heat removal systems, air condensers, etc. In such systems, the cooler (heavier) fluid naturally moves towards the bottom of the tank while the hotter lighter fluid rises towards the top. These flows result into certain extent of non-uniformity (or stratification) of temperature/concentration depending upon the intensity of flow. We have carried out flow (using particle image velocimetry) and temperature (using thermocouples) measurements in a rectangular tank (0.8×0.6×0.6m3) fitted with a central tube (forming the heat transfer surface). In order to reduce the stratification, the effects of various internals have been examined. These include (a) changing the ratio of area (heat transfer) to volume (aspect ratio), which is achieved by insulating the heating tube with the help of teflon, (b) introduction of draft tube concentric to the heat transfer tube, and (c) provision of non-conducting or highly conducting fins attached to the tube at different axial locations. Additionally, computational fluid dynamic (CFD) simulations of these systems were performed using the commercial software FLUENT-6.3. The extent of stratification has been investigated for a wide range of Rayleigh numbers (4.34×1011≤Ra≤2.59×1014). In addition, the flow information obtained from PIV was analyzed for insights into the dynamics of turbulent flow structures. For this purpose, we have used the signal processing technique of Discrete Wavelet Transform (DWT). From the analysis, we were able to estimate the size, velocity and energy distribution of turbulent structures. This detailed knowledge was employed in surface renewal type of theories for the estimation of rates of heat transfer. A good agreement was observed between the predicted and the experimental values of heat transfer coefficient. •Investigation of temperature and flow patterns in centrally heated cylindrical tank.•Effect of passive devices (aspect ratio, draft tube and fins) on stratification.•Conducting fins of suitable size with its appropriate locations severely reduces mixing time and stratification.