CONVECTION HEAT TRANSFER CASE STUDY

CONVECTION HEAT TRANSFER CASE STUDY 

INTRODUCTION
Convection involves the transfer of heat by the motion and mixing of "macroscopic" portions of
a fluid (that is, the flow of a fluid past a solid boundary). The term natural convection is used
if this motion and mixing is caused by density variations resulting from temperature differences
within the fluid. The term forced convection is used if this motion and mixing is caused by an
outside force, such as a pump. The transfer of heat from a hot water radiator to a room is an
example of heat transfer by natural convection. The transfer of heat from the surface of a heat
exchanger to the bulk of a fluid being pumped through the heat exchanger is an example of
forced convection.

Heat transfer by convection is more difficult to analyze than heat transfer by conduction because
no single property of the heat transfer medium, such as thermal conductivity, can be defined to
describe the mechanism. Heat transfer by convection varies from situation to situation (upon the
fluid flow conditions), and it is frequently coupled with the mode of fluid flow. In practice,
analysis of heat transfer by convection is treated empirically (by direct observation).
Convection heat transfer is treated empirically because of the factors that affect the stagnant film
thickness:

    -Fluid velocity
    -Fluid viscosity
    -Heat flux
    -Surface roughness
    -Type of flow (single-phase/two-phase)

CASE STUDY 
Convection involves the transfer of heat between a surface at a given temperature (Ts) and fluid
at a bulk temperature (Tb). The exact definition of the bulk temperature (Tb) varies depending
on the details of the situation. For flow adjacent to a hot or cold surface, Tb is the temperature
of the fluid "far" from the surface. For boiling or condensation, Tb is the saturation temperature
of the fluid. For flow in a pipe, Tb is the average temperature measured at a particular cross section
of the pipe.

The basic relationship for heat transfer by convection has the same form as that for heat transfer
by conduction:

     Q⠁  = h A 𝛥T

where:
Q⠁  = rate of heat transfer (Btu/hr)
h      = convective heat transfer coefficient (Btu/hr-ft²-F°)
A     = surface area for heat transfer (ft²)
𝛥T   = temperature difference (F°)
The convective heat transfer coefficient (h) is dependent upon the physical properties of the fluid
and the physical situation. Typically, the convective heat transfer coefficient for laminar flow
is relatively low compared to the convective heat transfer coefficient for turbulent flow. This is
due to turbulent flow having a thinner stagnant fluid film layer on the heat transfer surface.
Values of h have been measured and tabulated for the commonly encountered fluids and flow
situations occurring during heat transfer by convection.

Many applications involving convective heat transfer take place within pipes, tubes, or some
similar cylindrical device. In such circumstances, the surface area of heat transfer normally give in the convection equation ( Q⠁ = h A 𝛥T ) varies as heat passes through the cylinder. In addition,
the temperature difference existing between the inside and the outside of the pipe, as well as the
temperature differences along the pipe, necessitates the use of some average temperature value
in order to analyze the problem. This average temperature difference is called the log mean
temperature difference (LMTD), 

Logarithmic Mean Temperature Difference - LMTD

The mean temperature difference in a heat transfer process depends on the direction of fluid flows involved in the process. The primary and secondary fluid in an heat exchanger process may

◾flow in the same direction - parallel flow or co-current flow

◾ in the opposite direction - counter-current flow
◾ or perpendicular to each other - cross flow

In order to solve certain heat exchanger problems, a log mean temperature difference (LMTD

or )T ) must be evaluated before the heat removal from the heat exchanger is determined.

Convective Heat Transfer Coefficients

Convective heat transfer coefficients - hc - depends on type of media, if its gas or liquid, and flow properties such as velocity, viscosity and other flow and temperature dependent properties.
Typical convective heat transfer coefficients for some common fluid flow applications:
-Free Convection - air, gases and dry vapors : 0.5 - 1000 (W/(m²K))
-Free Convection - water and liquids: 50 - 3000 (W/(m²K))
-Forced Convection - air, gases and dry vapors: 10 - 1000 (W/(m²K))
-Forced Convection - water and liquids: 50 - 10000 (W/(m²K))
-Forced Convection - liquid metals: 5000 - 40000 (W/(m²K))
-Boiling Water : 3.000 - 100.000 (W/(m²K))
-Condensing Water Vapor: 5.000 - 100.000 (W/(m²K))

Heat Transfer Coefficients - Units
 ⅅ  1 W/(m²K) = 0.85984 kcal/(h m² ⁰C) = 0.1761 Btu/(ft² h ⁰F)
 ⅅ  1 Btu/(ft² h ⁰F) = 5.678 W/(m²K) = 4.882 kcal/(h m² ⁰C)
 ⅅ  1 kcal/(h m² ⁰C) = 1.163 W/(m²K) = 0.205 Btu/(ft² h ⁰F)

Convective Heat Transfer Chart

Convective Heat Transfer Coefficient for Air

The convective heat transfer coefficient for air flow can be approximated to 

h = 10.45 - v + 10 v¹⁄²

where
v = relative speed between object surface and air

Convection Heat Transfer Summary

• Convection heat transfer is the transfer of thermal energy by the mixing and motion of a fluid or gas.

• Whether convection is natural or forced is determined by how the medium is placed into motion.

• When both convection and conduction heat transfer occurs, the overall heat transfer coefficient must be used to solve problems.

• The heat transfer equation for convection heat transfer is    Q⠁ = h A 𝛥T