Studies on steam-air mixture condensation in a vertical tube to enhance heat transfer with/without thermal energy storage | |
| Author | Chanamon Chantana |
| Call Number | AIT Diss. no.ET-14-01 |
| Subject(s) | Heat storage--Mathematical models Condensation Heat--Transmission |
| Note | A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Energy |
| Publisher | Asian Institute of Technology |
| Abstract | Pure vapor condensation is an effective heat transfer mode due to its high heat transfer coefficient in a condenser. But, in presence of non-condensable gas, heat and mass transfer rate from vapor condensation is greatly reduced. Many theoretical models based on heat and mass transfer analogy have been proposed for a high fraction of inlet vapor fraction. In these models, the heat and mass analogy equations are modified with the effects that enhance heat and mass transfer rate during the condensation process. These effects, for example, include interfacial stress, rough and suction effects and thermal entrance length. However, there is no investigation of these effects to heat and mass transfer rate at high fraction of non-condensable gas, such as flue gas from natural gas fired boiler, where the air fraction is above 80%. Considering the above, theoretical and experimental studies were conducted to study the effect of non-condensable gas to heat and mass transfer in a vertical tube condenser. The first objective of the present study investigated the influence of enhanced effects on heat and mass transfer rate during water vapor condensation at high fraction of air (as non-condensable gas). This was done by conducting theoretical and experimental studies considering steam-air mixture.A mathematical model based on heat and mass transfer analogy was first developed to study the heat and mass transfer characteristics when water vapor condenses at high inlet air mass fraction. The condenser was a vertical tube type (2 m length) having two different diameter stainless steel pipes in a concentric configuration. The steam-air mixture flowed downward in the annulus and cold water as the coolant flowed upward in the inner tube. The steam mass fraction in steam-air mixture was in the range of 0.03 to 0.12, and the gas mixture Reynolds number was in the range of 4,600 to 14,000. The model predictions showed the effect of non-condensable gas in reducing the heat transfer coefficient and heat flux along the condenser tube length. It also showed that the Nusselt and Sherwood number increased when rough and suction effects were included. The interface temperature also decreased along the condenser tube and could be assumed to be the pipe wall temperature due to the low rate of vapor condensation. The increase in inlet water vapor mass fraction and steam-air mixture Reynolds number can increase Nusselt and Sherwood number and the interface temperature. An experimental set up was constructed with associated measuring devices. The observations and results from the experimental studies showed that the condensate film Reynolds number was in the laminar region. The measured water vapor condensation rates from the conducted experiment were compared with the predicted condensation rates from the model. The results of the comparison showed that including rough and suction effects in the model is necessary for accurate prediction. The model predictions also agreed well with the experimental data in literature. The results from the model showed that as inlet water vapor mass fraction is increased, the condensation heat transfer coefficient hcdhas more influence on the overall heat transfer coefficient than the sensible heat transfer coefficient hg. However, at lower inlet water mass fraction and gas mixture Reynolds number, the contribution of hg to the overall heat transfer coefficient could be more than 50 percent. The Nusselt correlation of condensation heat transfer coefficient was also proposed from the model and validated by the experimental studies. iv One technique used to increase heat transfer rate from a gas was by inserting packed bed into the gas flow passage. The packed bed increases the gas heat transfer coefficient and heat also can be stored. However, only few studies are available that considered steam-air mixture with a large fraction of air flowing and condensing in packed bed, where part of the heat transfers to coolant and some stores in packed bed. Thus, the second objective of this research was to conduct experiments and investigate the effect of packed bed to heat and water vapor condensation from steam-air mixture. Experiments with air flowing in packed bed was also conducted to compare the results between air flow in bare tube and steam-air mixture flow in packed bed. The experimental conditions of air/steam-air mixture in packed bed were the same as in bare tube test. The experimental results when air flowed in packed bed showed that the increasing air Reynolds number increased the temperature in packed bed. The air Nusselt number in packed bed was higher than that of bare tube by 1.5 to 2 times. The increased rate was lower when compared with the experimental results in literature for similar experimental conditions. The Nusselt number of wall to air heat transfer coefficient was also correlated using the experimental data. The experimental results of steam-air mixture flows in packed bed showed that packed bed can increase the water vapor condensation compared to that in bare tube. The increasing rates are 10% and 20% on average for inlet water vapor mass fractions of 12% and 5% respectively. The increased rate was lower when compared with that in literature for similar experimental conditions. This was due to the increasing packed bed temperature with time when gas mixture flowed in packed bed and this reduced the vapor condensation rate. Packed bed increases both sensible and latent heat (from water vapor condensation). Thus, the temperature in packed bed was higher than that when air flowed in packed bed. The temperature increased with inlet water vapor fraction and Reynolds number. The total heat transfer rate in packed bed was higher than that in bare tube, by about 2 times on average. At high condensation rate (at high heat transfer rate) the heat was stored in the packed bed than to heat the cold water, which was observed to be as high as 60 percent at 0.12 inlet water vapor mass fraction and at 14,000 Reynolds number. |
| Year | 2014 |
| Type | Dissertation |
| School | School of Environment, Resources, and Development (SERD) |
| Department | Department of Energy and Climate Change (Former title: Department of Energy, Environment, and Climate Change (DEECC)) |
| Academic Program/FoS | Energy Technology (ET) |
| Chairperson(s) | Kumar, Sivanappan |
| Examination Committee(s) | Athapol Noomhorm ;Salam, P. Abdul |
| Scholarship Donor(s) | Asian Institute of Technology Fellowshi |
| Degree | Thesis (Ph.D.) - Asian Institute of Technology, 2014 |