Week 4 - CHT Analysis on Exhaust port

CHT

1.1 Aim: Conjugate heat transfer analysis on exhaust port.

1.2 Introduction:

When both conduction and convection is occurs at the same time then it is called as conjugate heat transfer. A conjugate heat transfer is physics problem which shows the heat transfer between solid body to flowing body of fluid. This type of analysis is used to calculate the relevant solid and flow fluid heat transfer process.

There are three modes of heat transfers i.e. conduction, convection and radiation.

1.3 Theory

Fourier law of heat conduction: in the plane layer, heat conduction rate is proportional to the temperature difference and heat transfer area across the layer, but inversely proportional to the layer thickness.

Thermal conductivity is measured of material ability to conduct heat.

= - K A

• Sign indicates heat transfer in direction of decreasing temperature.

Thermal conductivity (k) is rate of heat transfer through a unit thickness of material per unit area per unit temperature difference.

It is measure of ability of material to conduct heat.

A high value for thermal conductivity indicates that material is a good heat conductor and low value indicates that material is poor heat conductor or insulation.

For example: thermocol, air, wood, plastic,polythene,foam,etc

= perfect heat sink

= core temperature relatively

= good insulator

Thermal diffusivity α =

The layer value of α will lead to faster heat dissipation in the material.

The material with higher value of k and lower value of

High value of k will result in faster energy transfer the material.

Low value if c means lesser amount of heat moving through the material will be used for raising the temperature and more of it will be available for transferring.

Newton law of cooling: it states that convective heat transfer is proportional to difference in temperature between an object and its surrounding.

= h A

Convection is a mode of energy transfer that involves the combined effects of conduction and fluid motion between solid surface and an adjacent moving liquid or gas.

Faster the fluid motion higher the convective heat transfer.

Convection heat transfer coefficient

The convection heat transfer coefficient h is not property of fluid.

It is an experimentally determined parameter whose values depends on variable influencing convection such as surface geometry, nature of fluid motion, properties of fluid, bulk fluid velocity.

Nusselt number: It is ratio of convective heat transfer to conductive heat transfer within a fluid region.

Nu =

Nu denotes increase in heat transfer through a fluid layer caused by convection, relative to conduction.

Within a fluid motionless (no bulk motion) conduction will be dominating mode of heat transfer.

When there is bulk motion, convection will be higher.

Prandtl number: ratio of momentum diffusivity to thermal diffusivity.

It compares the velocity boundary layer thickness to thermal boundary layer thickness.

Prandtl number for gas are about 1.

For water, it is in order of 10.

Pr =

2.1 Modelling approach

Here we used K-omega model for the analysis which is used to find the effect of heat transfer between extracted volume to the outer wall surface.

K-omega is used to capture the effect of turbulent flow conditions,

Y+ parameter

For getting better result for boundary layer+ parameter is greatly affected. And y+ value is a non-dimensional number from wall surface to first layer mesh.

As the y+ value increases, the thickness of boundary layer also increased, here we use y+ value is 0.06

3.1 Geometry

The geometry is imported in space claim and extract the fluid domain

3.2 Mesh

Here I used tetrahedron type of mesh.

Inside geometry is volume extracted from the pipe and giving him inflation layer separately.

Here y+ parameter is also defined

1. First layer thickness = 0.06 mm
2. No of layers = 10
3. Growth rate = 1.2

By using y+ calculator I find the values by importing the following values

1. Free steam velocity = 5 m/s
2. Density = 1.204
3. Characteristic length = 0.17m
4. First cell height = 0.06 mm

• boundary condition

firstly for the analysis, I defined the inlet (4 faces), outlet(1 face), external pipe wall and internal pipe wall.

1. In cell zones

Fff_exhaist_port_solid1= it is treated as a solid.

Fluid domain = it is treated as a fluid domain.

2. Boundary

Inlet velocity = 5 m/s

Temperature = 700 K

Outlet temperature = 300 K

External pipe wall heat transfer coefficient is 20 w/m2k

4.1 Results obtained

The calculated value for surface heat transfer coefficient are as follows

Sr. No.	Region	w/m^2k
1	External pipe wall	-18.94
2	Internal wall	-21.35
3.	Net	-6.3

5.1 Conclusion

Since all boundary condition are applied correctly therefore the solution obtained through simulation is also valid. When it is checked with analytical calculation the values are very much matched with the simulation result.

The accuracy is totally depended on how the boundary conditions are defined

There are two geometries defined inside the whole geometry i.e, solid domain and fluid domain. So accordingly for getting better result, we have to use share topology command so that mesh element of solid body is match with fluid region. (i.e element to element & node to node)

Fine mesh also affects the result, as the number of layers is increase, mesh count also increases, so getting again better result.