Immersed heater-to-bed heat transfer and hydrodynamics in multiphase fluidized beds with glass bead were investigated. Individual phase holdups and heat transfer coefficients of two (gas-liquid, liquid-solid) and three phase (gas-liquid-solid) fluidized bed have been determined in a 15.2 cm ID column fitted with an axially mounted cylindrical heater. Effects of gas flow rate (0-12 cm/s), liquid flow rate (0-16 cm/s) particle size (1.7-8.0 mm) and liquid viscosity (1.0-39.0 mPas) on the heat transfer coefficient were examined. Complementary studies were also performed on gas-coal slurry, coal slurry-solid and gas-coal slurry-solid fluidized beds. Air, water and coal-mineral oil mixture were used as a gas, a liquid and a slurry phases, respectively.
To examine the effects of liquid phase radial mixing on heat transfer in the bulk flow, the radial dispersion coefficients of liquid phase were determined in two (liquid-solid) and three (gas-liquid-solid) phase fluidized beds. A two resistance model was proposed to analyze the heat transfer mechanism in three phase beds. The bed height and individual phase holdups and/or bed porosity were determined from the static pressure drop methods. In gas-liquid flow beds, gas holdup decreased with increasing liquid flow rate and liquid phase viscosity, whereas it increased with gas flow rate. The gas holdup in three phase fluidized beds decreased with an increase in liquid phase viscosity. However, the liquid holdup decreased with increasing gas flow rate and particle size and increased with increasing liquid flow rate and liquid phase viscosity. The bed porosity, which is the indication of bed expansion, generally increased with increasing liquid and gas flow rates and liquid phase viscosity. However, effects of particle size on the bed porosity were dependent upon liquid phase viscosity and gas flow rate. Similar trends were observed in the beds of coal-slurry systems, since the coal-slurry could be regarded as the homogeneous phase with the same viscosity.
Heat transfer coefficients from the immersed heater to bed were determined from the knowledge of heat flux and the temperature difference between the heater and the beds. The heat transfer coefficients in gas-liquid and gas-coal slurry flow beds increased with gas flow rate and decreased with liquid (slurry) phase viscosity, however, an increase in liquid (slurry) flow rate produced minor effect on heat transfer coefficients. In liquid solid and coal slurry-solid systems, the heat transfer coefficients increased with an increase in particle size and decreased with liquid(slurry) phase viscosity. However, the coefficient exhibited the maximum value along with the liquid (slurry) flow rate and liquid (slurry) holdup or bed porosity. The bed porosity at which the heat transfer coefficient attained the maximum point decreased with particle size and increased with liquid phase viscosity. The optimum bed porosity at which the maximum heat transfer coefficient would occur coincided with the bed porosity when the energy consumption rate in the bed attained the maximum value. The heat transfer coefficient in three phase and three phase coal slurry fluidized beds were higher than those of comparable gas-liquid (coal slurry) and liquid (coal slurry)-solid systems except for small particle size at high gas flow rate. The coefficients in three phase (slurry) beds increased with increasing gas flow rate and particle size; however, it decreased with an increase in liquid (slurry) phase viscosity, whereas the coefficient exhibited its maximum value with a variation of the liquid (slurry) flow rate. The optimum bed porosity for maximum heat transfer was in the range of 0.55-0.70.
Liquid phase radial dispersion coefficients were estimated by employing the infinite space model. The radial dispersion coefficients of the continuous liquid phase increased with an increase in fluidizing particle size; however, the coefficients showed the maximum curve along with the liquid flow rates both in two (liquid-solid) and three (gas-liquid-solid) phase fluidized beds. Increasing the gas flow rate in three phase beds led to intensify the liquid radial mixing in all the experimental conditions. From the proposed two resistance model, heater zone resistance prevails over the resistance of the bed zone for heat transfer in three phase fluidized beds.
다상 유동층의 열전달계수 및 액상의 반경방향 확산 계수 그리고 각상의 상체류량을 측정하여 그 특성을 고찰하였으며, 기상 유속, 액상유속 유동 입자의 크기 및 액상의 점도가 열전달 계수, 상체류량 및 혼합도에 미치는 영향을 검토하였다.
공기, 물 및 석탄-오일 슬러리, 그리고 유리 구슬 등이 각각 기체, 액체 고체상으로 사용되었으며, 삼상 유동층의 열전달 현상을 설명하기 위하여 2-저항 모델이 제안, 적용되었다.
기체-액체 유동층에서, 액상 유속과 액상 점도가 증가함에 따라 기체의 상체류량은 감소하였으며, 삼상 유동층에선, 기상 체류량이 액상의 점도가 증가함에 따라 감소하였다.
삼상 유동층에서, 액상 체류량은 기상의 유속 및 유동입자의 크기가 증가함에 따라 감소하였으나, 액상의 유속 및 점도가 증가함에 따라 증가하였다. 삼상 유동층의 공극율은 일반적으로 액상 및 기상 유속, 그리고 액상의 점도가 증가할수록 증가하였으나, 유동입자의 크기에 의한 영향은 액상의 점도와 기상의 유속에 따라 변화하였다. 이와같은 일반적인 경향은 석탄-오일슬러리 유동층에서도 관찰할 수 있었다. 유동층 중앙의 열원과 유동층사이의 열전달 계수는 기체-액체 유동층에선 기상의 유속이 증가할수록, 액상의 점도가 감소할수록 증가하였으며, 액상 유속의 영향은 상당히 작았다. 액체-고체 유동층에서, 열전달 계수는 유동 입자의 크기가 증가할수록, 액상 점도가 감소할수록 증가하였으나, 액상 유속이 증가함에 따라 최대값을 보였고, 이열전달 계수가 최대값을 나타낼 때의 공극율은 유동입자의 크기가 증가할수록, 액상 점도가 감소할수록 감소하였다. 또한 열전달 계수를 최대로하는 공극율 값은 유동층 내에서의 역학적 에너지가 최대로 소비될 때의 공극율값과 일치하였다.
삼상 유동층에서의 열전달계수는 기체-액체 및 액체-고체 유동층의 경우 보다 큰 값을 나타냈으며, 기상 유속 및 유동입자의 크기가 증가할수록 증가하였고, 액상 점도가 증가할수록 감소하였으며, 액상 유속이 증가함에 따라 최대값을 나타내었다. 삼상 유동층에서 최대 열전달 계수를 나타낼 때의 공극율 범위는 0.55 ~ 0.70 이었다.
삼상 유동층에서 액상의 혼합도가 증가하면 열전달에 좋은 효과를 나타내었는데, 이 액상의 반경방향 혼합도는 유동입자의 크기가 증가할수록, 기상 유속이 증가할수록 증가하였으나, 액상 유속이 증가할수록 역시 최대값을 나타내었다.
2-저항 모델로 부터 삼상 유동층의 열전달을 열원 영역과 유동층 영역으로 나누어 생각할수 있고, 열원 영역의 열전달 저항이 유동층 영역의 열전 저항보다 지배적이었다.
