Week 4 - CHT Analysis on Exhaust port

AIM: Perform CHT Analysis on Exhaust port.

OBJECTIVES:

1. Give a brief description of why and where a CHT analysis is used.

2. Maintain the y+ value according to the turbulence model and justify the results.

3. Calculate the wall/surface heat transfer coefficient on the internal solid surface & show the velocity & temperature contours in appropriate areas.

4. How would you verify if the HTC predictions from the simulations are right? On what factors does the accuracy of the prediction depend on?

CHT Analysis:

The CHT (Conjugate Heat Transfer) Analysis is simulation done in order to understand the interactions that occurs when a fluid floors around (or through) a solid media.

This analysis gives information about how the heat is transferred at the solid – liquid interface. Since for solids heat flow predominantly occurs through conduction and for fluids it’s through convection.

This simulation gives a clear picture how heat will flow from the solid into the fluid or vice versa.

Applications off CHT Analysis:

CHT Analysis practically have uncountable applications, since in general, heat transfer between solids and fluids occurs almost everywhere. Nuclear, Automobile, thermal plants, Aerospace, etc. all sectors the CHT analysis finds itself useful one way or other.

Y+ Value:

Near the fluid – solid interface the flow characteristics are not same as seen in the remaining flow region. To capture that same flow characteristics, it is necessary to have elements of such order and capabilities near the interface region.

The idea about these near wall elements is obtained through a dimensionless number known as the Y+ value.

This value gives the mesh size values near the interface, the values can be calculated using formulas or can be obtained online by submitting the necessary values asked.

In the present problem, the exhaust gases will be flowing out from different outlet points and will mix together before exiting the exhaust manifold.

The exhaust gases flow and mixing will be modelled in this problem. The steps taken to do so are as follows:

Geometry:

Import the geometry.

Create the fluid volume.

Share the common edges of the fluid volume and the solid body.

Clean up the geometry if there are any geometrical irregularities.

Meshing:

Create Named Selections for inlets, outlet and solid body outer surface.

Generate the baseline mesh with default element size.

Check for the conformal mesh at the fluid – solid interface.

It is clear from the above figure that the mesh is conformal, that finishes the mesh generation step for the baseline model.

However, for the baseline model the inflation layers are not included since those will be added in the refined model in later stage.

Analysis setup:

The analysis setup for the problem is described as follows:

The analysis is conducted using K – Epsilon model.

Pressure based steady state solver is used. The materials for the solid volume and the fluid volume used are shown in figures below:

The inlet velocity is equal to 5 m/s and the temperature of the inlet air is equal to 700 K.

The convective heat transfer coefficient between the solid body outer layer and the fluid surrounding is taken as 20 W/m² K.

The same analysis setup is used for the refined model also, with only difference being addition of the inflation layers at the interface and refined mesh size for the solid body with size of 150 mm.

The output quantities from the analysis are:

Heat Transfer Coefficient between solid boundary inner wall and exhaust air flowing inside.

Air flow temperature and velocity.

The results from the both the models are as follows:

Heat Transfer Coefficient:

The converged value is seen to be 21.815 W/m²K.

Velocity:

The figure shows the maximum velocity is occurring at the bend region.

Temperature:

The temperature is more near the outlet after the mixing have taken place and the maximum temperature seen is equal to 575.8 K.

The results for the refined model are as follows:

Heat Transfer Coefficient:

Temperature:

The figure gives the wall adjacent temperature variation near the outlet pipe region and the maximum temperature is seen to be equal to 588.6 K.

Velocity:

The velocity variation is depicted in above figure, maximum velocity is seen around the bend region which is around 40 m/sec.

From both studies, heat transfer coefficient obtained remained around 21.8 W/m²K. this value can be verified through theoretical calculations for the given problem.

The Nusselt number mathematically is defined as:

Nu = 0.027 Re ^0.8 Pr^0.33 = (h * L)/K

Where,

Re = Reynolds Number = 56820.722 (for present case)

Pr = Prandlt Number = 0.74

L = Pipe diameter = 0.166m

K = Conductivity of fluid (Air) = 0.0242 W/m K

Using these values the analytical value of the Heat Transfer Coefficient came out to be around 19.6 W/m²K.

Both the values are close enough to verify that the study conducted is correct.