Free & Forced Convection Heat Transfer

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  • Topic: Heat transfer, Convection, Heat exchanger
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Experiment 8 - Free & Forced Convection Convection Heat Transfer.doc

EXPERIMENT ON FREE AND FORCED CONVECTION HEAT TRANSFER 8.1 OBJECTIVES To study experimental data for heat transfer in order to evaluate the overall heat transfer coefficients and heat balances for the following cases of heat transfer in a .shell and tube heat exchanger. (a) Natural convection and (b) Forced convection. 8.2 THEORY A basic diagram of a shell and tube heat exchanger is shown in Figure 8.1. Here steam at a temperature of Tv is sent to the shell side at the port A at a rate of W kg/s. The steam transfers heat to a fluid at the tube side .The steam condenses during this process and leaves the shell side at the port B at a temperature Ts. The tube side fluid enters the heat exchanger at C with a flow rate of M kg/s at a temperature Ti and leaves at D at a temperature To. The heat loss QH from the steam can be expressed as QH = W(λ + CpH.(Tv-Ts)) Similarly, the heat gained by the tube side fluid QC can be expressed as QC= M.CpT. (Ti-To) The heat transfer coefficient for the shell side and tube side hH and hc can be estimated using QH = hH .ΔTM and QC = hC. ΔTM .

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Experiment 8 - Free & Forced Convection Convection Heat Transfer.doc

STEAM AT TV ENTER AT PORT A

TUBE SIDE FLUID ENTERS AT Ti

TUBE SIDE FLUID LEAVES AT To

CONDENSED STEAM LEAVES PORT B AT TEMPERATURE TS

FIGURE 8.1 BASIC LAY-OUT OF A SHELL AND TUBE HEAT EXCHANGER
Calculation of the individual heat transfer coefficients involves the following dimensionless numbers: The Nusselt number, Nu . where α= individual heat transfer coefficient [W/m2·K] d = characteristic length [m] (e.g. tube diameter) λ = thermal conductivity [W/m·K] cp = specific heat [J/kg·K] μ = dynamic viscosity [Pas] v = velocity [m/s] ρ = density [kg/m3] The Prandtl number, Pr

The Reynolds number, Re

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Experiment 8 - Free & Forced Convection Convection Heat Transfer.doc

For natural convection one dimensionless number is added: The Grashof number, Gr

where g β ΔT = gravitational acceleration [m/s2] = volumetric expansion coefficient [K-1] = temperature difference [K]

. At natural convection the following generalized equation applies:

. At forced convection the following generalized equation applies:

. With dimensionless numbers and equations any consistent system of units can be applied. . The physical meaning of the different dimensionless numbers is as follows: Nu = (characteristic length)/(theoretical film thickness) Pr = (momentum diffusivity)/(thermal diffusivity) Re = (momentum by eddy diffusion)/(momentum by molecular transport) Gr = (inertia forces)/(viscous shear forces)·(buoyancy forces)/ (viscous shear forces) The following equation is valid for laminar flow conditions (Sieder & Tate):

. where α dh A s λ μb μw L Re Pr = individual heat transfer coefficient [W/m2·K] = hydraulic diameter [m] = 4·A/s = cross-sectional area [m2] = wetted perimeter [m] = thermal conductivity [W/m·K] = dynamic bulk viscosity [Pas] = dynamic wall viscosity [Pas] = tube length [m] = Reynolds number = Prandtl number

The equation is valid for:

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Experiment 8 - Free & Forced Convection Convection Heat Transfer.doc

A large number of other equations are also available in literature, for different fluids, flow conditions and geometries 8.3 APPARATUS The layout of the equipment is shown in Figure 8.2 The apparatus consists of a constant head glass feeder tank E (6”OD x 5 5/8” height) fitted with a vented aluminium top and bottom plate. The bottom plate is connected to a water inlet E1to the tank and also to a water overflow outlet E2 through a weir. A weir overflow drain pipe E3 is adjustable vertically through weir adjustment grip ring W, a packing gland assembly located near the middle of the line. Once the weir height is adjusted, the pointer indicates the level of the top of the weir from the zero position on the calibrated scale. The...
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