Generally Heat transfer coefficient of fluid increases with the increase in Reynolds number or velocity of fluid when it flows through a pipe. What parameters of fluid increases that causes an increase in heat transfer coefficient? What is the physical concept behind it?
Possible resons for higher Reynolds number:
Greater Velocity
Greater temperature
Greater density
Lower hydraulic diameter
For developed laminar flow, the velocity (or rather the mass flux) has no influence on the heat transfer coefficient. This is because tha radial transport by heat is by conduction only. For developing flow, the Reynolds number has an influence; this is because the velocity influences the length of the developing region (where the boundary layers grow).
For turbulent flow, the heat transfer coefficient increases with increasing mass flux (or Reynolds number). This is because higher velocity increases turbulence, which in turn provides a more efficient radial transport/mixing of heat.
When a viscous fluid flows in contact with a tube at low velocity it will do so in a way which does not produce any intermixing of the fluid: the boundary layer, the fluid in contact with the tube, will have its velocity reduced slightly by viscous drag and heat will flow through the fluid out of (or into) the tube wall by conduction and/or convection. As the velocity of the fluid is increased it will eventually reach a level which will cause the fluid to form turbulence eddies where the boundary layer breaks away from the wall and mixes with the bulk of the fluid further from the tube wall. The velocity at which this occurs is influenced by many factors, the viscosity of the fluid, the roughness of the tube wall, the shape of the tube, size of the tube etc. In order to quantify the turbulence (or lack of it) in practical terms heat transfer Engineers use a dimensionless number called the Reynolds number and it is equal to inertia forces to viscous forces of fluids.
In forced convection Reynolds number plays a major role. If the system is open , fluid velocity and fluid-metal surface contact area have significant effect in heat transfer
Starting from a general framework, the Reynolds number is a ratio between diffusive and convective flux of the momentum, apparently no link seems to exist. On the other hand, the Peclet number is nothing else that Pr*Re and it appears in the internal energy equation.
In natural convection is more correct to refer as to the Rayleigh number and several correlation were proposed. Considering forced convection, you shoud start from the concept that h= q/ Delta T where q depends on the normal derivative of temperature at the wall. The Re number define the BL thickness and the dynamic and thermal properties in the BL depends on the position, that is nothing else that the local Re number.
A brief summary can be found everywhere in internet, for example http://www.sfu.ca/~mbahrami/ENSC%20388/Notes/Forced%20Convection.pdf
One good explanation you can find on:
http://physics.stackexchange.com/questions/64120/what-does-the-reynolds-number-of-a-flow-represent-physically
A turbulent flow leads to greater particle friction that also increases the heat transfer in contrast to the laminar flows.
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