A detailed comparative numerical study between the two-dimensional（2 D） and quasi-two-dimensional（quasi-2 D）turbulent Rayleigh–B＇enard（RB） convection on flow state, heat transfer, and thermal dissipation rate...A detailed comparative numerical study between the two-dimensional（2 D） and quasi-two-dimensional（quasi-2 D）turbulent Rayleigh–B＇enard（RB） convection on flow state, heat transfer, and thermal dissipation rate（TDR） is made. The Rayleigh number（Ra） in our simulations ranges up to 5×10^10 and Prandtl number（Pr） is fixed to be 0.7. Our simulations are conducted on the Tianhe-2 supercomputer. We use an in-house code with high parallelization efficiency, based on the extended PDM–DNS scheme. The comparison shows that after a certain Ra, plumes with round shape, which is called the ＂temperature islands＂, develop and gradually dominate the flow field in the 2 D case. On the other hand, in quasi-2 D cases, plumes remain mushroom-like. This difference in morphology becomes more significant as Ra increases, as with the motion of plumes near the top and bottom plates. The exponents of the power-law relation between the Nusselt number（Nu） and Ra are 0.3 for both two cases, and the fitting pre-factors are 0.099 and 0.133 for 2 D and quasi-2 D respectively,indicating a clear difference in magnitude of the heat transfer rate between two cases. To understand this difference in the magnitude of Nu, we compare the vertical profile of the horizontally averaged TDR for both two cases. It is found that the profiles of both cases are nearly the same in the bulk, but they vary near boundaries. Comparing the bifurcation height zb with the thermal boundary layer thickness dq, it shows that zb 〈 δθ（3 D） 〈 δθ（2 D） and all three heights obey a universal power-law relation z ~Ra^-0.30. In order to quantify the difference further, we separate the domain by zb, i.e., define the area between two zb（near top and bottom plates respectively） as the ＂mid region＂ and the rest as the ＂side region＂, and integrate TDR in corresponding regions. By comparing the integral it is found that most of the difference in TDR between two cases, which is connected to the heat transfer rate, occurs within the thermal boundary layers. We also compare the ratio of contributions to total heat transfer in BL–bulk separation and side–mid separation.展开更多
This paper describes the experimental study of viscosity measurement error in the vibrational type viscometer, which measures viscous damping of the oscillating circular plate in a fluid in continuously increasing tem...This paper describes the experimental study of viscosity measurement error in the vibrational type viscometer, which measures viscous damping of the oscillating circular plate in a fluid in continuously increasing temperatures. The experiments are carried out to measure non-uniformity of the temperature field in the test cup of the vibrational viscometer in continuously increasing temperatures, while changing the viscosity of the target fluids. Experimental outcomes show that non-uniformity of the temperature grows in the cup and results in viscosity measurement error, when the viscosity of the fluid increases. In order to understand this phenomenon, velocity measurement by particle image velocimetry is conducted in the test cup for fluids of varying viscosity. The results indicate that mixing is enhanced in the low-viscosity fluid by the occurrence of unsteady thermal convection, while weaker convection appears in the high-viscosity fluid.展开更多
Calibration of radiation thermometers is one of the important research activities in the field of metrology. Many researchers in recent times have conducted numerical simulations on the calibration furnace to understa...Calibration of radiation thermometers is one of the important research activities in the field of metrology. Many researchers in recent times have conducted numerical simulations on the calibration furnace to understand and overcome the experiment limitations. This paper presents a 2D numerical free convective study on the calibration furnace with the aperture using finite element method. The focused issues here are: aspect ratio effect on the flow pattern and temperature fields, heat transfer mechanism in the aperture zone as well as in hump regime. It is concluded that flow and temperature fields follow the same behavior in the hump regime as well as in the aperture zone. Also, it concluded that penetrative convection is more dominant for the enclosure of high aspect ratio.展开更多
In geothermal systems, the temperature distribution of heat flow in the wellbore is dependent on the well structure and the geological conditions of the surrounding formation. Understanding of heat transfer in the tub...In geothermal systems, the temperature distribution of heat flow in the wellbore is dependent on the well structure and the geological conditions of the surrounding formation. Understanding of heat transfer in the tubing-casing annulus can reduce the heat losses of wellbore fluid during the production process. The present study discusses the possible means of heat transfer in the annulus, and develops a piecewise equation for estimating the convective heat transfer coefficient with a wider valid condition of 0〈Ra〈7.17×10^8. By converting the radiation and natural convection into equivalent thermal conduction, their sum is defined as a total thermal conductivity to describe the heat transfer in the annulus. The results indicate that the annulus filled with gas can be utilized as a good thermal barrier for the fluid in the wellbore. Additionally, the contribution of radiation will increase to occupy a majority proportion in the total thermal conductivity when the annular size increases and the materials have high emissivity. Otherwise, thermal radiation is just the second factor.展开更多
The present paper employs the direct simulation Monte Carlo (DSMC) method to study the Rayleigh-Bénard flows, where the temperature ratio of the upper to lower plate is fixed to 0.1. For a Knudsen number (Kn) of ...The present paper employs the direct simulation Monte Carlo (DSMC) method to study the Rayleigh-Bénard flows, where the temperature ratio of the upper to lower plate is fixed to 0.1. For a Knudsen number (Kn) of 0.01, as the Rayleigh number (Ra) increases, the flow changes from the thermal conductive state to the convective state at about Ra=1700, and the calculated relation of heat flux through the lower plate versus Ra is in good agreement with classical experimental and theoretical results. For Kn=0.05, the thermal conductive state remains stable, and the increase of Ra cannot trigger thermal instability.展开更多
基金Project supported by the National Natural Science Foundation of China(Grant Nos.11772362 and 11452002)the Special Scientific Research Fund for Super Computing in the Joint Fund of the National Natural Science Foundation of Chinathe People’s Government of Guangdong Province(Phase Ⅱ,Grant No.nsfc2015 570)
文摘A detailed comparative numerical study between the two-dimensional（2 D） and quasi-two-dimensional（quasi-2 D）turbulent Rayleigh–B＇enard（RB） convection on flow state, heat transfer, and thermal dissipation rate（TDR） is made. The Rayleigh number（Ra） in our simulations ranges up to 5×10^10 and Prandtl number（Pr） is fixed to be 0.7. Our simulations are conducted on the Tianhe-2 supercomputer. We use an in-house code with high parallelization efficiency, based on the extended PDM–DNS scheme. The comparison shows that after a certain Ra, plumes with round shape, which is called the ＂temperature islands＂, develop and gradually dominate the flow field in the 2 D case. On the other hand, in quasi-2 D cases, plumes remain mushroom-like. This difference in morphology becomes more significant as Ra increases, as with the motion of plumes near the top and bottom plates. The exponents of the power-law relation between the Nusselt number（Nu） and Ra are 0.3 for both two cases, and the fitting pre-factors are 0.099 and 0.133 for 2 D and quasi-2 D respectively,indicating a clear difference in magnitude of the heat transfer rate between two cases. To understand this difference in the magnitude of Nu, we compare the vertical profile of the horizontally averaged TDR for both two cases. It is found that the profiles of both cases are nearly the same in the bulk, but they vary near boundaries. Comparing the bifurcation height zb with the thermal boundary layer thickness dq, it shows that zb 〈 δθ（3 D） 〈 δθ（2 D） and all three heights obey a universal power-law relation z ~Ra^-0.30. In order to quantify the difference further, we separate the domain by zb, i.e., define the area between two zb（near top and bottom plates respectively） as the ＂mid region＂ and the rest as the ＂side region＂, and integrate TDR in corresponding regions. By comparing the integral it is found that most of the difference in TDR between two cases, which is connected to the heat transfer rate, occurs within the thermal boundary layers. We also compare the ratio of contributions to total heat transfer in BL–bulk separation and side–mid separation.
文摘This paper describes the experimental study of viscosity measurement error in the vibrational type viscometer, which measures viscous damping of the oscillating circular plate in a fluid in continuously increasing temperatures. The experiments are carried out to measure non-uniformity of the temperature field in the test cup of the vibrational viscometer in continuously increasing temperatures, while changing the viscosity of the target fluids. Experimental outcomes show that non-uniformity of the temperature grows in the cup and results in viscosity measurement error, when the viscosity of the fluid increases. In order to understand this phenomenon, velocity measurement by particle image velocimetry is conducted in the test cup for fluids of varying viscosity. The results indicate that mixing is enhanced in the low-viscosity fluid by the occurrence of unsteady thermal convection, while weaker convection appears in the high-viscosity fluid.
文摘Calibration of radiation thermometers is one of the important research activities in the field of metrology. Many researchers in recent times have conducted numerical simulations on the calibration furnace to understand and overcome the experiment limitations. This paper presents a 2D numerical free convective study on the calibration furnace with the aperture using finite element method. The focused issues here are: aspect ratio effect on the flow pattern and temperature fields, heat transfer mechanism in the aperture zone as well as in hump regime. It is concluded that flow and temperature fields follow the same behavior in the hump regime as well as in the aperture zone. Also, it concluded that penetrative convection is more dominant for the enclosure of high aspect ratio.
文摘In geothermal systems, the temperature distribution of heat flow in the wellbore is dependent on the well structure and the geological conditions of the surrounding formation. Understanding of heat transfer in the tubing-casing annulus can reduce the heat losses of wellbore fluid during the production process. The present study discusses the possible means of heat transfer in the annulus, and develops a piecewise equation for estimating the convective heat transfer coefficient with a wider valid condition of 0〈Ra〈7.17×10^8. By converting the radiation and natural convection into equivalent thermal conduction, their sum is defined as a total thermal conductivity to describe the heat transfer in the annulus. The results indicate that the annulus filled with gas can be utilized as a good thermal barrier for the fluid in the wellbore. Additionally, the contribution of radiation will increase to occupy a majority proportion in the total thermal conductivity when the annular size increases and the materials have high emissivity. Otherwise, thermal radiation is just the second factor.
基金Supported by the National Natural Science Foundation of China (Grant Nos. 90205024, 10502051 and 10621202)
文摘The present paper employs the direct simulation Monte Carlo (DSMC) method to study the Rayleigh-Bénard flows, where the temperature ratio of the upper to lower plate is fixed to 0.1. For a Knudsen number (Kn) of 0.01, as the Rayleigh number (Ra) increases, the flow changes from the thermal conductive state to the convective state at about Ra=1700, and the calculated relation of heat flux through the lower plate versus Ra is in good agreement with classical experimental and theoretical results. For Kn=0.05, the thermal conductive state remains stable, and the increase of Ra cannot trigger thermal instability.
