Proceeding of Proceedings of the 26thNational and 4th International ISHMT-ASTFE Heat and Mass Transfer Conference December 17-20, 2021, IIT Madras, Chennai-600036, Tamil Nadu, India, 2022
We report the non-intrusive investigation of the dependence of buoyancy-driven flow instabilities... more We report the non-intrusive investigation of the dependence of buoyancy-driven flow instabilities on the orientation angle of a differentially heated cavity of aspect ratio three. The cavity orientation angles considered are 60° and 30°. While moving from 60° to 30°, the cavity is inclined toward its stable configuration, wherein convection reduces. Flow instabilities have been captured through the spectral analysis of the transient history of temperature distribution recorded in a completely non-intrusive manner using a Mach–Zehnder interferometer. By virtue of the fact that in such configurations, corners of the cavity are the most active regions with regard to the interaction of buoyancy-driven fluid with the cavity walls, and the flow behavior is centrosymmetric (diagonal symmetry), the flow field in the top two corners of the cavity has been mapped. The spatio-temporally resolved interferometric measurements identified two distinct frequencies for cavity inclination angle (θ) of 60°. These two frequencies correspond to two different flow instabilities, namely, the Tollmien–Schlichting (TS) and gravity wave-induced instabilities. As the cavity is further inclined toward 30°, the instability in the boundary layer, i.e., the TS instability, ceases to exist, and only the gravity wave-induced instability is observed. The dependence of flow instabilities on cavity orientation angle is explained on the basis of interferometry-based measurements made in the form of interferograms and the corresponding whole field maps of temperature contours. The convective flow field in the differentially heated cavity has also been qualitatively captured using smoke visualization to provide direct support to interferometric measurements.
Proceeding of Proceedings of the 26thNational and 4th International ISHMT-ASTFE Heat and Mass Transfer Conference December 17-20, 2021, IIT Madras, Chennai-600036, Tamil Nadu, India, 2022
We report the non-intrusive investigation of the dependence of buoyancy-driven flow instabilities... more We report the non-intrusive investigation of the dependence of buoyancy-driven flow instabilities on the orientation angle of a differentially heated cavity of aspect ratio three. The cavity orientation angles considered are 60° and 30°. While moving from 60° to 30°, the cavity is inclined toward its stable configuration, wherein convection reduces. Flow instabilities have been captured through the spectral analysis of the transient history of temperature distribution recorded in a completely non-intrusive manner using a Mach–Zehnder interferometer. By virtue of the fact that in such configurations, corners of the cavity are the most active regions with regard to the interaction of buoyancy-driven fluid with the cavity walls, and the flow behavior is centrosymmetric (diagonal symmetry), the flow field in the top two corners of the cavity has been mapped. The spatio-temporally resolved interferometric measurements identified two distinct frequencies for cavity inclination angle (θ) o...
Journal of Flow Visualization and Image Processing, 2020
In this paper, a natural convection study of an open square cavity is carried out in order to und... more In this paper, a natural convection study of an open square cavity is carried out in order to understand the heat transfer and the fluid flow inside the cavity. The rectangular cavity considered in this work has a square cross-section and is made of two adiabatic walls and an isothermal wall. The thermally active wall is maintained at a relatively higher temperature than the ambient temperature. Experiments are performed for three different Rayleigh numbers for five different inclinations (from side wall open to top wall open) with air (Pr= 0.71) as the working fluid. A non-intrusive optical technique (Mach-Zehnder Interferometer) is employed to capture the depth-averaged two-dimensional steady-state temperature field in the cavity. In addition to that, smoke visualization is also carried out to see the flow behavior inside the cavity. The physics of fluid flow has been discussed with the help of interferograms, corresponding twodimensional temperature field, and smoke visualization results. The losses due to natural convection at different orientations have been quantified and are presented in terms of Nusselt numbers on the thermally active wall. The average heat transfer rates on the thermally active wall are found to be dependent on the orientation of the cavity. As the cavity is rotated such that the open face moves from the side to top, it is noted that the average Nusselt number decreases. However, when the opening of the cavity is at the top, there is a sudden increment in heat transfer due to Rayleigh-Benard convection in the cavity. The experimentally obtained average Nusselt numbers have been compared with those obtained by numerical simulations and good agreement between these two results is seen. The study reveals that higher driving potential for the higher angle of inclination led to higher heat transfer.
Natural convection in closed cavities has been a subject of intensive research in the past. Compa... more Natural convection in closed cavities has been a subject of intensive research in the past. Compared to numerical studies, the number of experimental works reported in this area have been relatively scarce. Of the limited number of experimental studies available in the literature, a majority have made use of invasive probes, which inherently disturb the flow. In the present work, real-time experimental measurements are carried out using one of the non-invasive techniques (Mach Zehnder interferometry), that provides the whole field temperature distribution of the fluid layer. Experiments are conducted in a differentially-heated vertical closed cavity of aspect ratio three with air as the working fluid. The vertical side walls of the cavity have been subjected to three temperature differences (ΔT = 10, 20 and 30 °C) (Ra = 9.7 × 10 5 , 1.8 × 10 6 and 2.5 × 10 6 ). Transient numerical simulations have also been performed using COMSOL Multiphysics 5.2 and a detailed comparison of experimental and simulation results has been presented in the form of temperature contours, spatial distribution of Nusselt number and spatially-averaged heat transfer rates as a function of Rayleigh number. The interferometric measurements highlighted the importance of corner flows which affect the heat transfer rates between the two thermally active walls of the cavity. Buoyancy-induced flow patterns inside the cavity, as interpreted through interferometric measurements, have further been corroborated through smoke-based visualization technique as well as through the results of numerical simulations. Maximum heat transfer rates have been observed in the corners of the differentially heated cavity. Possible flow transitions have been captured by performing the spectral analysis of the interferometry-based transient data. Based on this analysis, Ra = 1.8 × 10 6 was found to be greater than the critical Rayleigh number wherein the flow instabilities with two dominant frequencies were to be clearly seen.
The onset of natural convection in a 2D air filled cavity open at the top with adiabatic side wal... more The onset of natural convection in a 2D air filled cavity open at the top with adiabatic side walls is studied. The numerical model shows the existence of weak convective flow near the top corner of a cavity due to the thermal gradient between the walls and the atmosphere even at low Rayleigh numbers, as also confirmed by the interferometry-based experimental data. Additionally, a thermally stratified layer is formed on the lower side of the cavity. The onset of convection is seen to be dependent on the interaction of these two features in the cavity. Results of the study show that in low aspect ratio cavities, the thermally stratified layers are clearly formed and are not significantly disturbed by the flow at the corners. The onset of convection takes place in these earlier thermally stratified layers beyond a certain Rayleigh number. This convective movement is characterized by a sudden jump in the heat transfer coefficient at a critical Rayleigh number. However, for high aspect ...
Proceeding of Proceedings of the 26thNational and 4th International ISHMT-ASTFE Heat and Mass Transfer Conference December 17-20, 2021, IIT Madras, Chennai-600036, Tamil Nadu, India, 2022
We report the non-intrusive investigation of the dependence of buoyancy-driven flow instabilities... more We report the non-intrusive investigation of the dependence of buoyancy-driven flow instabilities on the orientation angle of a differentially heated cavity of aspect ratio three. The cavity orientation angles considered are 60° and 30°. While moving from 60° to 30°, the cavity is inclined toward its stable configuration, wherein convection reduces. Flow instabilities have been captured through the spectral analysis of the transient history of temperature distribution recorded in a completely non-intrusive manner using a Mach–Zehnder interferometer. By virtue of the fact that in such configurations, corners of the cavity are the most active regions with regard to the interaction of buoyancy-driven fluid with the cavity walls, and the flow behavior is centrosymmetric (diagonal symmetry), the flow field in the top two corners of the cavity has been mapped. The spatio-temporally resolved interferometric measurements identified two distinct frequencies for cavity inclination angle (θ) of 60°. These two frequencies correspond to two different flow instabilities, namely, the Tollmien–Schlichting (TS) and gravity wave-induced instabilities. As the cavity is further inclined toward 30°, the instability in the boundary layer, i.e., the TS instability, ceases to exist, and only the gravity wave-induced instability is observed. The dependence of flow instabilities on cavity orientation angle is explained on the basis of interferometry-based measurements made in the form of interferograms and the corresponding whole field maps of temperature contours. The convective flow field in the differentially heated cavity has also been qualitatively captured using smoke visualization to provide direct support to interferometric measurements.
Proceeding of Proceedings of the 26thNational and 4th International ISHMT-ASTFE Heat and Mass Transfer Conference December 17-20, 2021, IIT Madras, Chennai-600036, Tamil Nadu, India, 2022
We report the non-intrusive investigation of the dependence of buoyancy-driven flow instabilities... more We report the non-intrusive investigation of the dependence of buoyancy-driven flow instabilities on the orientation angle of a differentially heated cavity of aspect ratio three. The cavity orientation angles considered are 60° and 30°. While moving from 60° to 30°, the cavity is inclined toward its stable configuration, wherein convection reduces. Flow instabilities have been captured through the spectral analysis of the transient history of temperature distribution recorded in a completely non-intrusive manner using a Mach–Zehnder interferometer. By virtue of the fact that in such configurations, corners of the cavity are the most active regions with regard to the interaction of buoyancy-driven fluid with the cavity walls, and the flow behavior is centrosymmetric (diagonal symmetry), the flow field in the top two corners of the cavity has been mapped. The spatio-temporally resolved interferometric measurements identified two distinct frequencies for cavity inclination angle (θ) o...
Journal of Flow Visualization and Image Processing, 2020
In this paper, a natural convection study of an open square cavity is carried out in order to und... more In this paper, a natural convection study of an open square cavity is carried out in order to understand the heat transfer and the fluid flow inside the cavity. The rectangular cavity considered in this work has a square cross-section and is made of two adiabatic walls and an isothermal wall. The thermally active wall is maintained at a relatively higher temperature than the ambient temperature. Experiments are performed for three different Rayleigh numbers for five different inclinations (from side wall open to top wall open) with air (Pr= 0.71) as the working fluid. A non-intrusive optical technique (Mach-Zehnder Interferometer) is employed to capture the depth-averaged two-dimensional steady-state temperature field in the cavity. In addition to that, smoke visualization is also carried out to see the flow behavior inside the cavity. The physics of fluid flow has been discussed with the help of interferograms, corresponding twodimensional temperature field, and smoke visualization results. The losses due to natural convection at different orientations have been quantified and are presented in terms of Nusselt numbers on the thermally active wall. The average heat transfer rates on the thermally active wall are found to be dependent on the orientation of the cavity. As the cavity is rotated such that the open face moves from the side to top, it is noted that the average Nusselt number decreases. However, when the opening of the cavity is at the top, there is a sudden increment in heat transfer due to Rayleigh-Benard convection in the cavity. The experimentally obtained average Nusselt numbers have been compared with those obtained by numerical simulations and good agreement between these two results is seen. The study reveals that higher driving potential for the higher angle of inclination led to higher heat transfer.
Natural convection in closed cavities has been a subject of intensive research in the past. Compa... more Natural convection in closed cavities has been a subject of intensive research in the past. Compared to numerical studies, the number of experimental works reported in this area have been relatively scarce. Of the limited number of experimental studies available in the literature, a majority have made use of invasive probes, which inherently disturb the flow. In the present work, real-time experimental measurements are carried out using one of the non-invasive techniques (Mach Zehnder interferometry), that provides the whole field temperature distribution of the fluid layer. Experiments are conducted in a differentially-heated vertical closed cavity of aspect ratio three with air as the working fluid. The vertical side walls of the cavity have been subjected to three temperature differences (ΔT = 10, 20 and 30 °C) (Ra = 9.7 × 10 5 , 1.8 × 10 6 and 2.5 × 10 6 ). Transient numerical simulations have also been performed using COMSOL Multiphysics 5.2 and a detailed comparison of experimental and simulation results has been presented in the form of temperature contours, spatial distribution of Nusselt number and spatially-averaged heat transfer rates as a function of Rayleigh number. The interferometric measurements highlighted the importance of corner flows which affect the heat transfer rates between the two thermally active walls of the cavity. Buoyancy-induced flow patterns inside the cavity, as interpreted through interferometric measurements, have further been corroborated through smoke-based visualization technique as well as through the results of numerical simulations. Maximum heat transfer rates have been observed in the corners of the differentially heated cavity. Possible flow transitions have been captured by performing the spectral analysis of the interferometry-based transient data. Based on this analysis, Ra = 1.8 × 10 6 was found to be greater than the critical Rayleigh number wherein the flow instabilities with two dominant frequencies were to be clearly seen.
The onset of natural convection in a 2D air filled cavity open at the top with adiabatic side wal... more The onset of natural convection in a 2D air filled cavity open at the top with adiabatic side walls is studied. The numerical model shows the existence of weak convective flow near the top corner of a cavity due to the thermal gradient between the walls and the atmosphere even at low Rayleigh numbers, as also confirmed by the interferometry-based experimental data. Additionally, a thermally stratified layer is formed on the lower side of the cavity. The onset of convection is seen to be dependent on the interaction of these two features in the cavity. Results of the study show that in low aspect ratio cavities, the thermally stratified layers are clearly formed and are not significantly disturbed by the flow at the corners. The onset of convection takes place in these earlier thermally stratified layers beyond a certain Rayleigh number. This convective movement is characterized by a sudden jump in the heat transfer coefficient at a critical Rayleigh number. However, for high aspect ...
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Papers by VIMAL KISHOR