Exhaust Port Challenge

Q1. Why and Where a CHT analysis is used?

Conjugate Heat Transfer analysis:-

Conjugate heat transfer analysis is generally used when there is a teperature variation during heating and cooling in the material due to interaction between solid and fluid phases. It is the combination of heat transfer in solids to heat transfer to fluids.

Examples of conjugate heat transfer analysis:-

HVAC, Heat exchanger, Tank, Internal combustion engine, Heat sink, Boiler, Reactor, Heat pipe, Turbocharger.

Benefits:-

Conjugate Heat transfer analysis is beneficial in accurately predicting the surface temperatures and heat exchanging capabilities of engineering devices. Through the analysis engineers can optimize the thermal characteristics for their choosen product design.

It can be used for following reasons:-

1. Measurements of heat transfer through conduction in solid regions.
2. Identification of fluid flow parameters around the solid domain.
3. Measurement of convective heat transfer fro solids to surrounding fluid enviornment.
4. Evaluation of heat transfer capabilities of the product.
5. measurement of surface temperatures and hotspot regions.
6. Design optimization for improvement in heat transfer rate and cooling capacity.

Details of some applications:-

Turbocharger:-

The figure below depicts the various sources, sink and paths in diesel engine turbocharger. Turbocharger which comprises of turbine stage and compressor stage has various sources of heat generation and these requires cooling, which is carried away by means of convective heat transfer where fluid such as oil, coolant or air is circulated. CHT analysis can be used to calculate compressor heat efficiency value and estimate the quantities of heat which enter and leave the respective stages both before and after compression. The image below show the CHT model of a cummims non water cooled HE200VG turbocharger, generated in order to investigate compressor stage performace at several running points when considering the heat transfer across the housings and core components and to carry out comparision of adiabatic and non adiabatic compressor performance using CHT analysis.

Shell and Tube type heat exchanger:-

Shell and tube type heat exchangers are most commonly employed heat exchangers in industry where one fluid runs inside the tubes and and another fluid runs over the tube and heat transfer takes place.

CHT analysis of heat exchngers allows along with CFD allows:-

1. Effictive prototyping during heat exchanger design phase.
2. Visualization of flow patterns and alter the geometry of the HEx to suit the temperature requirements.
3. Enhance baffle designs to reduce system pressure drop, choosing an optimized shape and weight for the shell casing.
4. Simulation of complex processes such as condensation and vapourisation inside the system.

Applications

`N_u = (h times l)/k`

Automotive radiators and oil coolers.

Powerplant components such as Boiler pressure parts, air cooled condensers and cooling towers.

Condesers and evaporators in refrigeration units.

Nuclear reactors.

Q2 CHT analysis on a model of an exhaust manifold.:-

In the following project CHT analysis on exhaust manifold is carried out to find out velocity, temperature and heat transfer coefficient. Two simulations with baseline mesh and refined mesh are carried out and compared.

GEOMETRY CREATION-

i) Geometry of Exhaust Manifold:- Imported exhaust manifold geometry in space claim.This is the solid body model of the Exhaust port manifold. The material for this model is aluminum.

ii) Extracting Fluid Volume by selecting edges:- In order to define the fluid volume 4edges of inlets and one edge of outlet is selected.

iii) Extracted Fluid Volume:- This is the extracted fluid volume where the fluid flows. The given below figure represents the fluid region inside the Exhaust port manifold. Consider the 4 ports as inlet combining to a single outlet. The velocity of the inlet fluid is 5m/s with temperature of 700k.

iv) Fluid Volume inside Solid Volume:-

v) Sharing Topology of Geometry:- In order to effectively capture the Conjugate Heat Transfer between the solid & fluid regions, both solid & fluid regions should share the properties which could be done by enabling the Share Topology option. The pink coloured edges and yellow faces shows which information is being shared with both solid and fluid volume.

In the above figure the edges with pink colour and faces with yellow colour shows that the edges and faces are sharing the information correctly. This also work to obtain the confirmal mesh.

Case1: Baseline Mesh

Named Selection:-

Baseline Mesh:-

Element size - Default 0.15 m

No. of Nodes - 27631

No. of Elements - 138655

Confirmal Mesh:- Faces and vortex are coinciding perfectly.

Mesh Quality:-

Setup:-

Pressure based solver with absolute velocity formulation k-epsilon viscous model. Energy equation is turned on.

Boundary Conditions:-

Inlet - Velocity is 5m/sec with 700 K temperature.

Outlet - Pressure outlet

Outer wall-convection - Thermal convection with Heat Transfer coefficient 200 `W/m^2 K`

With the above baseline mesh and solver setup. Hybrid initialization is done and calculation is ran for 1000 iterations and obtain the results.

Scaled Residuals:-

The above figure shows us the velocity, energy & continuity curve with respect to the number of iterations.

Temperature Contours:-

The maximum temperature found in the throat region this is due to the high heat transfer occurs because of the fluid velocity at the throat region is high.

Velocity Contours:-

As observed it tends to increase where there is sharp curve and zero at the boundaries due to the law of the wall boundary condition. The maximum velocity found here is 37.835 m/s at the throat region.

Stream Lines:-

No restrictions found between the flow path when coloured with velocity, it seemed to increase near the exhaust.

Contours of Wall Heat Transfer Coefficient:-

The Wall Heat Transfer Co-efficient is higher at the lower radius throat region for about 100.149 W/K.m^2 this occurs due to more number hot fluid molecule passes along the region at a higher velocity which helps to transfer the heat from the fluid at a higher rate.

Case 2: Refined Mesh

Named Selection:-

Mesh -

In this case a refined mesh is setup with finer mesh with body sizing in solid section and inflation layers near convective heat transfer region.

Basic element size - 0.015 m

Body sizing in solid zone with element size - 0.015 m

Inflation Layer - No. of layers - 5

Total thickness with maximum thickness - 0.0005 m

No. of Nodes - 169056

No. of Elements - 478888

Inflation Layers -

No. of layers - 5

Total thickness with maximum thickness - 5 mm

Mesh Quality:-

Setup:- Pressure based solver with absolute velocity formulation k-epsilon viscous model. Energy equation is turned on.

Boundary Conditions:-

Inlet - Velocity is 5 m/sec with 700 K temperature.

Outlet - Pressure outlet

Outer wall-convection - Thermal convection with Heat Transfer coefficient 200 `W/m^2 K`

With the above refined mesh and solver setup. Hybrid initialization is done and calculation is ran for 1500 iterations and obtain the results.

Scaled Residuals

The above figure shows us the velocity, energy & continuity curve with respect to the number of iterations.

Temperature Contours:-

The maximum temperature found in the throat region this is due to the high heat transfer occurs because of the fluid velocity at the throat region is high.

Velocity Contour:-

As observed it tends to increase where there is sharp curve and zero at the boundaries due to the law of the wall boundary condition. The maximum velocity found here is 41.481 m/s at the throat region.

Stream Lines:-

No restrictions found between the flow path when coloured with velocity, it seemed to highest near the exhaust.

Contours of Wall Heat Hransfer Coefficient:-

The Wall Heat Transfer Co-efficient is higher at the lower radius throat region for about 221.657 W/K.m^2 this occurs due to more number hot fluid molecule passes along the region at a higher velocity which helps to transfer the heat from the fluid at a higher rate.

Observation:-

PARAMETERS	CASE 1	CASE 2
MINIMUM WALL TEMPERATURE(K)	464.286	504.06
MAXIMUM VELOCITY	37.83	41.48
WALL HEAT TRANSFER COEFFICIENT (`W/m^2 K`)	100.149	221.657

1. We can infer that refined mesh produces more accurate results near the walls due to proper interpolation.
2. The heat transfer coefficient calculate in case-2 has huge difference as compared to case-1.

Q3 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 heat transfer coefficient is a quantitative characteristic of convective heat transfer between a fluid medium and the surface medium flowed over by the fluid. The overall heat transfer coefficient for a multi-layered wall, pipe or heat exchanger - with the fluid flow can be calculated using Nusselt number formula,

`N_u = (h times D)/k`

where,

k= the thermal conductivity of the wall material (W/(m·K))

h= convection heat transfer coefficient (W/m^2.K)

D= inner diameter of the pipe (m)

We know that the thermal conductivity for aluminum is k=205 W/mK. The inner diameter & the outer diameter of the pipe is known. The value for the Nusselt number can be found using the Dittus-Boelter equation.

`N_u = 0.023 times (R_e)^(4/5) times (P_r)^0.4`

Using the above formulas we can verify the output of the solver & also we can verify the computational result with the experimental results.

The accuracy of the Heat Transfer Co-efficient depends on the inbound surface wall layer thickness, refined mesh & a converged solution.

Factors on which accuracy of prediction depends on:-

1. Number of elements in the mesh:- Finer mesh gives more accurete results.
2. Inflation layers around the boundary:- Inflation layers which provide body fitted mesh which provide accurate analysis of geometry.
3. Accurate fitting of mesh around the geoetry:- Mesh needs to accurately accomodate the geometry around the curvatures.
4. Setting share topolygy:- Mesh should be conformal for smooth transferring of mesh in different Zones.