Measurement and computation of drag forces in thermogravimetric studies
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(1) Sometimes, a precise initial weight inside is desirable. This requires correcting the weight in the thermogravimetric apparatus for drag. (2) For powder samples, sometimes some powder escapes from the crucible upon starting the gas flow. This can be determined properly if the drag is known.
of the drag force, generally speaking, constitutes a part of the fundamental understanding for such a setup. In this context, reference can be made to the measurements of liquid velocities in metallic melts through the drag probes.[1] It is through the precise knowledge of drag forces experienced by the measuring probes in such a system that inferences on melt velocities can be made. An experimental program has been undertaken to determine the carbonate capacity of Na2O-SiO2-B2O3 melts.[2] The temperature range was 1000 ⬚C to 1250 ⬚C. The melt was contained in a platinum crucible, which was hung from a Cahn recording balance and was introduced into the hot zone of the furnace under a purified argon gas flow. Then, the argon flow was stopped and a dry CO2 gas flow was started. The solubility of CO2 in the melt was determined from the weight gain due to absorption of the same by the melt. The carbonate capacity was calculated from the solubility data. Since the study of carbonate capacity has already been published,[3] it will not be discussed here further. However, during the course of the aforementioned investigation, experimental data were also collected on the upward drag force exerted by the flowing gases (argon and carbon dioxide) at various temperatures and different gas flow rates. The present article reports these findings. The flow was also simulated numerically, and the drag forces were computed with the help of the commercial computational fluid-dynamics (CFD) package called “FLUENT.”[4] Finally, direct comparison between model predictions and experimental measurements is presented. The article also shows calculations by an analytical method, assuming flow past the sphere, and compares the same with the results of numerical simulation.
Hence, investigators should have some awareness of the drag behavior in thermogravimetry. Moreover, knowledge
II. EXPERIMENTAL
DIPAK MAZUMDAR, Professor, and AHINDRA GHOSH, Ex Professor, are with the Department of Materials and Metallurgical Engineering, Indian Institute of Technology, Kanpur, 208016, India. Contact e-mail: [email protected] KAMLESH K. SINGH, Lecturer, is with Foundry Technology, National Institute of Foundry and Forge Technology, Ranchi 834 003, India. Manuscript submitted January 21, 2002.
The experimental apparatus consisted of a platinum-rhodium–wound vertical tube furnace, a Cahn 1000 automatic recording electrobalance along with a strip-chart recorder for continuous recording of the weight change of the sample as a function of time, and a gas train to feed dry and pure gases at known and controlled flow rates. The balance had a sensitivity of 10 g. A schematic of the thermogravimetric
THE thermogravimetric technique has been widely employed to study vari
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