Conjugate Heat Transfer analysis of Exhaust Port

Conjugate Heat transfer Analysis:

Conjugate heat transfer analysis is the simulation of the heat transfer between Solid and Fluid domains by exchanging thermal energy at the interfaces between them it is generally used when there is temperature variation during heating or cooling in the material due to the interaction between solid and fluid phases.

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 the engineering devices. Through the analysis, engineers can optimize the thermal characteristics for their chosen product design.

It can be used for the following reasons:-

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

Applications :

Shell and Tube type Heat exchanger:

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

Turbocharger:

The figure below depicts the various heat sources and paths in engine turbochargers. Turbocharger which comprises of turbine stage and compressor stage has various sources of heat generation and these require cooling, which is carried away 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.

CHT analysis of Heat exchangers allows along with CFD allows

1. Effective Prototyping during the HeatExchanger 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 vapourization inside the system.

CHT Analysis of 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.

CASE-1. BASELINE MESH.

A Baseline mesh with 138261 elements is created.

Analysis Set-Up:

The analysis is done using Energy method, K-ε viscous model with air as fluid domain and aluminum as solid domain

Input Parameters:

1. The temperature of fluid: 800 K
2. The velocity of Fluid at Inlet: 6 m/s
3. convective Heat Transfer Coefficient of the wall: 20 W/m2-K
4. other walls are kept as adiabatic and outlet at atmospheric pressure

Results for a baseline mesh :

1. Baseline mesh Residuals

2. Temperature Contour

3. Velocity Plot

4. Streamline plot

The velocity is maximum at outlet and minimum at inlet this shows that it is due to the fact of mass conservation A1V1=A2V2 since inlet and outlet has same dia and all the fluid from the inlet exits at the same outlet, hence the velocity is maximum at outlet since all the fluid tries to flow through the one outlet hence mass conservation plays a role.
The Maximum velocity is seen at the end of the manifold which is around 44 m/s

5. Wall Heat Transfer co-efficient Plot

Due to an increase in velocity at the outlet Reynolds number increases, so the convective heat transfer coefficient increased since forced convection involves Nusslet number which is dependent on Re and Pr hence the heat transfer is higher at the outlet and especially where velocity is higher.
The heat transfer occurs at the walls as shown above and it is maximum at the bend where velocity is higher

CASE 2 REFINED MESH

In the following case, a refined mesh is setup with finer mesh in the solid section and
inflation layers in the inlet openings. Around 475876 elements are created.

Analysis Set-Up:

The analysis is done using Energy method, K-ε viscous model with air as fluid domain and aluminum as solid domain

Results for Refined Mesh:

Temperature contour

Velocity plot

Streamline plot :

Wall Heat transfer coefficient plot :

Observations:

PARAMETERS CASE-1 CASE-2

Wall Temperature (K) 800 800

Max Velocity (m/s) 44.10 48.10

Wall Heat Transfer
Coefficient ( W-m /K) 117.9 240.02

1. From the above results, 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 a huge difference as compared to case-1
   

VERIFICATION OF HEAT TRANSFER COEFFICIENT :

It is verified with the help of Nusselt number since Nu=h l/K,

where,

h-the heat transfer co-eff,

l - Characteristic length,

K-thermal conductivity,

since Nu is dependent on Reynolds number Re and Prandtl number Pr we can get the Nu value, Nu=Re Pr.

If velocity is increased Re is increased which in turn increases the Nusslet number and
obviously it increases the HTC.

Factors affecting the accuracy of prediction :

1. Geometry/Dimensions (length,diameter etc.,)
2. Number of elements in the mesh
3. Properties of fluid and its velocity