Heat transfer gas-particle

Gas to particle heat transfer coefficients are t3q ically small, of the order of 5-20 W m K. However, because of the very large heat transfer surface area provided by a mass of small particles (1 m of 100 pm particles has a surface area of 60 000 m ), the heat transfer between gas and particles is rarely limiting in fluid bed heat transfer. One of the most commonly used correlations for gas-particle heat transfer coefficient is that of Kunii and Levenspiel (1969)  

Gas-particle heat transfer is relevant where a hot fluidized bed is fluidized by cold gas. The fact that particle-gas heat transfer presents little resistance in bubbling fluidized beds can be demonstrated by the following example  

Consider a fluidized bed of solids held at a constant temperature Tg. Hot fluidizing gas at temperature Tgo enters the bed. At what distance above the distributor is the difference between the inlet gas temperature and the bed solids temperature reduced to half its original value  

The distance over which the temperature difference is reduced to half its initial value, Lo.5, is then 

Using the Baeyens equation for Umf [Equation (7.11)], Umf = 9.3 x lO m/s. The relative velocity between particles and gas under fluidized conditions can be approximated as Umf/e under these conditions. 

---

It is also interesting to note that soft-sphere models have also been applied to other applications such as gas-particle heat transfer by Li and Mason (2000) and coal combustion by Zhou et al. (2003). Clearly, these methods open a new way to study difficult problems in fluidized bed reactors. 

In considering heat transfer in gas-solid fluidization it is important to distinguish between, on the one hand, heat transfer between the bed and a heat transfer surface (be it heated bed walls or heat transfer coils in the bed) and, on the other hand, heat transfer between particles and the fluidizing gas. Much of the fluidization literature is concerned with the former because of its relevance to the use of fluidized beds as heterogeneous chemical reactors. Gas-particle heat transfer is rather more relevant to the food processing applications of fluidization such as drying, where the transfer of heat from the inlet gas to the wet food particle is crucial. 

Kunii and Levenspiel (1991) summarised the experimentally determined values of fhe gas-particle heat transfer coefficient If, and the data are shown in Figure 2.1. At Re > 100 the value of Nu lies between that for a single particle and a fixed bed and these authors suggest that the relevant correlations are those due to Ranz, equations 2.9 and 2.10 respectively ... 

Figure 2.1 Experimental gas-particle heat transfer coefficients. Adapted from Kunii, D. and Levenspiel, O., Fluidization engineering, 1991, with permission from Elsevier.

Kunii and Levenspiel (1991) identify two kinds of heat transfer coefficient to describe gas-particle heat transfer. The coefficient for a single particle, or local coefficient. If, is that pertaining to a single particle at high temperature Tj, introduced suddenly into a bed of cooler particles at a temperature and is defined by... 

Table 2.1 Example calculation of gas-particle heat transfer coefficients.

Persson (1967) measured the variation in gas-particle heat transfer coefficient with particle diameter. Figure 3.5 shows the heat transfer coefficient for both an unspecified maximum gas velocity and for Fr = 120. For Fr = 120, h falls from approximately 130 Wm K at a diameter of 3mm to lOOWm K at a diameter of 16mm, these particle sizes corresponding to superficial gas velocities of 1.88ms and 4.33ms respectively. For larger diameters the heat transfer coefficient becomes approximately constant at 85Wm K . ... 

Figure 3.5 Variation in gas-particle heat transfer coefficient with particle diameter (A = fluidized bed at maximum air velocity B = fluidized bed at Fr = 120 C = flat surface velocity as for curve B). Adapted from Persson, ASHRAE Journal, June 1967. American Society of Heating, Refrigerating and Air-Conditioning...

Vazquez, A. and Calvelo, A., Gas particle heat transfer coefficient in fluidized pea beds, /. Food Proc. Eng., 4 (1980) 53-70. 

S.C.S. Rocha, Contribution to the study of pneumatic drying simulation and influence of the gas-particle heat transfer coefficient, Ph.D. Thesis, Sao Paulo University, Sao Paulo, Brazil, (in Portuguese), 1988. 

The governing heat transfer modes in gas-solid flow systems include gas-particle heat transfer, particle-particle heat transfer, and suspension-surface heat transfer by conduction, convection, and/or radiation. The basic heat and mass transfer modes of a single particle in a gas medium are introduced in Chapter 4. This chapter deals with the modeling approaches in describing the heat and mass transfer processes in gas-solid flows. In multiparticle systems, as in the fluidization systems with spherical or nearly spherical particles, the conductive heat transfer due to particle collisions is usually negligible. Hence, this chapter is mainly concerned with the heat and mass transfer from suspension to the wall, from suspension to an immersed surface, and from gas to solids for multiparticle systems. The heat and mass transfer mechanisms due to particle convection and gas convection are illustrated. In addition, heat transfer due to radiation is discussed. 

Littman, H. and Sliva, D. E. (1971). Gas-Particle Heat Transfer Coefficient in Packed Beds at Low Reynolds Number. In Heat Transfer 1970, Paris-Versailles, CT 1.4. Amsterdam ... 

The term-aipi ( ) at the molecular temperature transport equation (4.334) accounts for the gas-particle heat transfer, which is closed by [89, 97] ... 

---