The blue line in the graph is for a smooth tube while the red one is for a corrugated tube. The physical properties of the fluid are the determining factors for the heat transfer in this area which is inefficient in heat transfer terms. µ = the viscosity of the fluid (kg/m.s)īy experimentation, it has been found that Reynolds numbers of less than 2000 describe the condition at which there is no breaking away from the tube wall which is termed laminar flow.D = the hydraulic diameter of the tube (m).In order to quantify the turbulence (or lack of it) in practical terms heat transfer Engineers use a dimensionless number called the Reynolds number which is calculated as follows: 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. 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.Īs 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. Importantly the final values are heavily influenced by what happens at the level of the boundary layers, the fluid actually in contact with the heat transfer surface. The resistance to heat flow resulting from and, (designated the partial heat transfer coefficients) depend greatly on the nature of the fluids but also, crucially, on the geometry of the heat transfer surfaces they are in contact with. The values to be used for and are usually specified by the client as the result of experience while the designer will select the tube size, thickness and materials to suit the application. The outside “boundary layer” formed by the fluid flowing in close contact with the outside surface of the tube.The fouling layer formed by deposition of solids or semi-solids on the outside surface of the tube (which may or may not be present).The thickness of the tube wall and the material used will govern the resistance to heat flow though the tube itself.The fouling layer formed by deposition of solids or semi-solids on the inside surface of the tube (which may or may not be present).The inside “boundary layer” formed by the fluid flowing in close contact with the inside surface of the tube.The resistance to the heat flow is formed by 5 layers as follows: The driving force for heat transfer is the difference in temperature levels between the hot and cold fluids, the greater the difference the higher the rate at which the heat will flow between them and the designer must optimise the temperature levels at each stage to maximise the total rate of heat flow. One of the important factors controlling heat transfer is the resistance to heat flow through the various “layers” that form the barrier between the two fluids.

Turbulent vs laminar flow