I want to know in which applications we have to use film temperature and bulk temperature?
In fluid Mechanics, the bulk temperature, or the average fluid bulk temperature, is a convenient reference point for evaluating properties related to convective heat transfer, particularly in applications related to flow in pipes and ducts.
The concept of the bulk temperature is that adiabatic mixing of the fluid from a given cross section of the duct will result in some equilibrium temperature that accurately reflects the average temperature of the moving fluid, more so than a simple average like the film temperature.
Refs:
1. Kreith, Frank, and Bohn, Mark S. Principles of Heat Transfer, Sixth Edition.
2. Incropera & DeWitt Fundamentals of Heat and Mass Transfer, 6th Ed
3. http://www.multitherm.com/technical_articles_bulk_film_temp_impact.html#
In fluid Mechanics, the bulk temperature, or the average fluid bulk temperature, is a convenient reference point for evaluating properties related to convective heat transfer, particularly in applications related to flow in pipes and ducts.
The concept of the bulk temperature is that adiabatic mixing of the fluid from a given cross section of the duct will result in some equilibrium temperature that accurately reflects the average temperature of the moving fluid, more so than a simple average like the film temperature.
Refs:
1. Kreith, Frank, and Bohn, Mark S. Principles of Heat Transfer, Sixth Edition.
2. Incropera & DeWitt Fundamentals of Heat and Mass Transfer, 6th Ed
3. http://www.multitherm.com/technical_articles_bulk_film_temp_impact.html#
In case of single phase flow the bulk temperature used for calculation of heat flux is the mean temperature of fluid and the duct wall and in two phase the mean of saturation temperature of the fluid and the wall temperature is taken to obtain properties for calculations.
The bulk temperature ( sometimes called the mixing cup temperature) in a bounded convective thermal flow is something that is readily defined. Using Latex notation it is
_b=\frac{1}{Q}\int_A T \pmb{v}\cdot\pmb{n}dA
where T is the local temperature, Q is the volumetric flow rate through the cross-sectional area A with unit normal n, and v is the velocity vector. Or in words it is the integral of the local temperature times the component of velocity normal to the surface A , divided by the volumetric flow rate through that area.
The film temperature can mean different things for different circumstance ( e.g., Aria Tsam has given a link to to a gas liquid interface where the film temperature is referred to the interface temperature. )
In confined convective thermal flows, the film temperature is defined as the temperature of the heated surface, usually related to a solid surface bounding the flow. This means that the film temperature is always greater than the bulk temperature (in the absence of heat generation in the fluid). The film temperature depends on both the heat flux q_s at the surface and the local heat transfer coefficient h and is calculated by
q_s=h*A_s(T_f-_b)
where A_s is the surface area in contact with the fluid,T_f is the film temperature and _b is the bulk temperature
The film temperature often limits the duty of a heater, because if the film temperature is higher than the operating temperature of the fluid, fluid properties can degrade. The film temperature can be controlled by limiting q_s or by controlling h by varying the fluid velocity.
You can think it is natures gift allowing the transfer of heat from the conduction mode to convective mode of heat transfer and vice-versa, Since convection mode requires a fluid the heat sets it into motion and and the concept of boundary layer also comes int picture. This simple B.L complicates the transfer to such an extent that theoretical determination alone on the heat transfer co-efficient is inadequate. The Bulk temperature is just outside the B.L
bulk temperature is the average temperature of fluid flowing through a channel
film temperarure is the average temperature of surface and itsEnvironment tempearature
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