Experimental Study of heat Transfer between the Shallow Fluidized bed and a Tube Bundle Immersed in it

This paper presents an experimental study of heat transfer between a shallow fluidized bed and the surface of a single horizontal tube and a tube bundle, which is immersed in it. Carbon, which is prepared from the Date stones, is used as a solid to be fluidized and a compressed air as an external fluid. The results showed that the overall heat transfer coefficient of tubes bundle, which immersed in fluidized bed is lower than single tube. The modified correlations and analysis are offered by experimental work, which shows the relations between heat transfer coefficient and superficial mass velocity. Key word: heat transfer heat exchangers fluidized bed heat exchangers


Introduction:
Fluidized beds are widely used in industry for mixing solid particles with gases or liquid. In most industrial applications, a fluidized bed consists of a vertically oriented column filled with granular material, and a fluid (gas or liquid) is pumped up ward through a distributor at the bottom of the bed. When the drag force of flowing fluid exceeds gravity, the particles are lifted and fluidization occurs [1]. At very low speed flows the air will pass between the particles without disturbing their position. The bed is then called static bed and acts like a filter. By increasing the airflow to a higher level, some particles in the bed will start to move depending on the uneven air distribution in the bed. Then when bed's depth more than 50cm, it is called deep bed [2], but if the depth is beds less than 50 cm, it is called a shallow bed. At higher air flows the particles will be separated from each other and the bed volume increases. This is called incipient fluidization. Higher contact areas between air and the particles will be the result, which is considered an advantage during combustion [3]. Even at higher airflow the particles will be mixed with each other and the bed is more like a boiling fluid. Full fluidization. This is the domain where most advantages with fluidization can be found.
Continuous increase of the air flow means bigger size of the air bubbles and at the end these will occupy the whole width of the bed [3].
A uniform fluidization, which is the most desirable regime of operation of industrial fluidized bed, is prone to instabilities. As the fluid flow increases, bubbles of clear fluid are formed at the bottom of the bed and these bubbles travel to the surface.
Several measurements have been reported for heat transfer to or from a tube immersed horizontally in a fluidized bed [2][3][4][5][6][7][8]. The effects of fluidizing velocity, particle size, shape and density, tube diameter and the bed temperature on the heat transfer coefficient were investigated. It was observed that the value of the total heat transfer coefficient increase with increase in the bed temperature, decrease in the tube diameter, and increase in the air mass velocity.
Because of the heat transfer between the bed and immersed surface is a complicated process, despite the efforts of study for more than 20 years, there is no satisfactory model for the prediction of bed to surface heat transfer coefficient applicable even that the experimental data is empirically for practical design use.

Experimental apparatus and procedure:
The experimental apparatus used in this study has been described and shown as Fig. 1 . The experimental column is of rectangular cross-section (300*175mm), the bottom plate is perforated distributor. For the single tube test, the tube is arranged horizontally of 100mm height from the distributor, tube diameter is 25mm; for the tube bundle test, the tubes arranged in three layers and the lowest layer of three tubes is put in the height 85mm from the distributor. The middle layer is of two tubes are in the height of 130mm from the distributor and the top layer of three tubes is in the height of 185mm from the distributor. That is means the distance between the different layers at in vertical direction is 50mm. In the horizontal direction, the distance between the lateral tubes is 50mm as shown in Fig.2. Therefore the vertical and horizontal relative pitches are 2, the ratio of horizontal pitch to tube diameter is called the relative horizontal pitch, and similarly the relative vertical pitch is also defined.
The air is supplied by the blower (2890r.p.m/ 3 phase / 7.5 kW )for atmospheric fluidization and the orifice is used for measuring the flow air. The manometer is used for measuring the pressure drop in fluidized bed. The heater chamber consists of a spiral ceramic core of 10mm diameter, with winding electrical resistance wire ( 1 kW / 0.5 mm diameter) for heating the tube by A.C. In the temperature measurement system, there are 12 pairs of thermocouples fitted on the surface of each tube, 8 pairs on the tube center and 2 pairs for ends. In the positions of 50 mm higher and lower than the centerline of heating tube, there are thermocouples installed for measuring the bed temperature.
The fluidized particles used are of carbons, which were pre prepared from the date stones, were ground to irregular shapes.   Fig (2

) The Distance between the Different Layers
The average diameter is calculated as follows; The shape factor of particle calculated by [9]: 1 s And volume concentration for packed bed as [10] : Where the Bulk density calculated:

Results and analysis:
The over all heat transfer coefficient is obtained and is variation its shown with superficial mass velocity in figures (3 ,4 ). This means that the heat transfer coefficient of single tube is higher than tube bundle under the same condition. The interstitial space is no good for the particles to play the role as the heat "carrier" and also the gas is more stagnant to heat transfer.
It shows that the heat transfer coefficient of the top layer has the smallest magnitude; those of the bottom layer has the largest magnitude and those of the middle layer is just in between.
The reason for these changes is probably due to the interstitial space effect of the movement of particle, and the temperature difference of different layers of tube bundle and its depth, for heat transfer is also joining the effect.
The experiment results are useful, as the tube bundle is beneficial to be used in preparing carbon for industrial applications, due to the direct touch between solid particles and fluid. The benefit of the increased heating surface would be compensated by the decrease of heat transfer and the drop of temperature difference between the bed and immersed heating surface. Therefore the optimum design for these two compensating factors is interesting and important.