Week 4 - CHT Analysis on Exhaust port

AIM:

To develop a numerical setup for conjugate heat transfer analysis on exhaust port.

OBJECTIVE:

• Give a brief description of why and where a CHT analysis is used
• Maintain the y+ value according to the turbulence model and justify the results.
• Calculate the wall/surface heat transfer coefficient on the internal solid surface and show the velocity and temperature contours in appropriate areas
• How would you verify if the HTC predictions from the simulations are right? On what factors does the accuracy of the prediction depend on?

INTRODUCTION:

CONJUGATE HEAT TRANSFER:

The Conjugate Heat Transfer (CHT) analysis type allows for the simulation of heat transfer between solid and fluid domains by exchanging thermal energy at the interfaces between them. Typical applications of this analysis type exist as, but are not limited to, the simulation of heat exchangers, cooling of electronic equipment, and general-purpose cooling and heating systems. For example, heat sinks are optimized to combine heat transfer by conduction in the heat sink with the convection in the surrounding fluid.

Heat Transfer by Solids and Fluids

Heat Transfer in a Solid

In most cases, heat transfer in solids, if only due to conduction, is described by Fourier’s law defining the conductive heat flux, q, proportional to the temperature gradient:`q = -ktriangleT` .

For a time-dependent problem, the temperature field in an immobile solid verifies the following form of the heat equation:

`rho c_p((delT)/(delt)) = triangle.(ktriangleT)+q`

Heat Transfer in a Fluid

Due to the fluid motion, three contributions to the heat equation are included:

1. The transport of fluid implies energy transport too, which appears in the heat equation as the convective contribution. Depending on the thermal properties on the fluid and on the flow regime, either the convective or the conductive heat transfer can dominate.
2. The viscous effects of the fluid flow produce fluid heating. This term is often neglected, nevertheless, its contribution is noticeable for fast flow in viscous fluids.
3. As soon as a fluid density is temperature-dependent, a pressure work term contributes to the heat equation. This accounts for the well-known effect that, for example, compressing air produces heat.

APPLICATIONS OF CHT:

Heat exchangers, HVAC, Air-cooled and liquid cooled engines, Turbochargers, Cooling of nuclear reactors, Cooling towers etc..

MODELLING APPROACH:

PREPROCESSING:

GEOMETRY:

• Initially the exhaust port geometry is imported into the spaceclaim and the fluid volume is extracted from the domain.

• The solid and fluid volume are combined together as a single geometry by moving to a new component.
• Under workbench select share tool to share the information between the solid and fluid volume.

• This prevents fluent from generating a non-conformal mesh.

MESHING:

NAMED SELECTION:

The boundary names are defined by using the named selection or press N in the keyboard. The inlet, outlet, outer wall convection and the inflation layers are the boundary names defined in the project.

MESH:

• Mesh is created with the element size of 0.15m with a element quality selected under mesh metric to check the quality of mesh.

• The inflation layer is created for interior wall with 5 layers

• Then the body sizing is done for outer walls of 0.01m and capture curvature is set to yes.

• Number of nodes – 112395
• Number of elements – 371573

MESHED MODEL:

SETUP:

GENERAL:

• Pressure based solver
• Steady state condition

MODEL:

• Energy – on
• Viscous – k-epsilon

MATERIAL:

• Fluid – air
• Solid – aluminium

BOUNDARY CONDITION:

INLET:

Velocity inlet – 5m/s

Temperature – 700k

OUTER WALL:

Convection – heat transfer coefficient – 20 w/m2k

OUTLET:

Pressure outlet – 0 gauge pressure.

OTHER BOUNDARIES:

Heat flux – 0 w/m2 [adiabatic condition]

INITIALIZATION:

Hybrid initialization

The calculation is runned for 150 iterations.

RESULTS:

RESIDUAL:

TEMPERATURE CONTOUR:

VELOCITY CONTOUR:

From this plot we can say that velocity is high near the bend and is lower at the entry of outlet.

PRESSURE CONTOUR:

From this plot we can say that pressure is less at the region where the velocity is high satisifying the bernoulli's priciple.

TEMPERATURE STREAMLINE:

HEAT TRANSFER COEFFICIENT:

From the above plot we can say that the heat transfer coefficient is high near the bend that is where the velocity is high.

WALL ADJACENT TEMPERATURE:

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

The typical flow conditions in automotive exhaust system produces re in the range 10^3 to 5*10^4. The exhaust flow often enters the region re<2300 especially in exhaust manifold runners. In spite of that the flow remains actually turbulent since it has flowed through a substantial restriction. The exhaust valves and the persisting unsteady flow pulsation effects don’t favour the transistion to the laminar region.

According to the sieder tate relation which correlates the Nu number with Re and Pr

Nu = 0.027Re^0.8Pr^1/3

It means Nusselt no. is the function of Reynolds no and Prandtl number

As the turbulent boundary layer in the bend of the outlet port is thin maximum velocity of flow is observed at bend and hence higher Reynolds number.

Now, Nu = h*L/k

As length of pipe and thermal conductivity are constant value of h will increases as Nu increases and Nu increases as Re increases.

As per our result, heat transfer coefficient is high for higher value of velocity and hence our results are correct.

FACTORS AFFECTING THE QUALITY OF PREDICTION:

• More refined mesh leads to the better results
• Use inflation layers and apply boundary conditions
• Proper selection of turbulence model according to Y+ values.

CONCLUSION:

In this study CHT analysis has done to find temperature rise at the manifold because of hot air passing through it and we predicted the heat transfer coefficient as well as velocity contours were created to get the insight of fluid path and maximum velocity area.

It is found that heat transfer coefficient is more near the wall where the exhaust pipe bends.