Conjugate Heat Transfer(CHT) on model of Exhaust Port

Aim:

To Perform a CHT analysis on a model of an exhaust manifold and to compare the solutions for 2 sizes of meshes.

To Show the velocity, temperature and heat transfer coefficient plots and explain if refining the mesh produced any changes to the results.

The Conjugate Heat Transfer(CHT):-

Heat transfer in solids and heat transfer in fluids are combined in the majority of applications. This is because fluids flow around solids or between solid walls and because solids are usually immersed in a fluid.

An accurate description of heat transfer modes, material properties, flow regimes, and geometrical configurations enables the analysis of temperature fields and heat transfer. Such a description is also the starting point for a numerical simulation that can be used to predict conjugate heat transfer effects or to test different configurations in order, for example, to improve thermal performances of a given application.

The CHT approach has benefits for those applications where heat transfer is either non-uniform or difficult to calculate empirically. Not only that, but conjugate heat transfer analysis can accurately predict heat transfer by simultaneous solving all the relevant solid and flow field heat transfer processes.

The analysis type Conjugate heat transfer (CHT) allows the simulation of the heat transfer between Solid and Fluid domains by exchanging thermal energy at the interfaces between them.

In solids, conduction often dominates whereas, in fluids, convection usually dominates. Conjugate heat transfer is observed in many situations. For example, heat sinks are optimized to combine heat transfer by conduction in the heat sink with the convection in the surrounding fluid. Other examples include simulation of heat exchangers, and general-purpose cooling and heating systems.

Examples of conjugate heat transfer analysis:-

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

The outcome of CHT:

• Temperature distribution analysis.
• Heat flux rate analysis – calculation of wall heat flux rates or heat transfer coefficients.
• Visualization of heat transfer features for improved understanding of system flow phenomena.
• Identification of localized ‘hot spots’ or heat sinks in checking against specified requirements.
• Evaluating variation in temperature peaks for different operating conditions/ failure modes.
• Assessing insulation or heat shield effectiveness and optimizing of material selections.
• Coupling CHT with FE analysis to evaluate thermally induced stresses, expansion or fatigue.
• Temperature field analysis
• CFD thermal analysis
• Heat transfer coefficient analysis
• Heat transfer rate analysis

Geometry setup:

• It is essential to make sure both the solid part and the extracted volume that is the fluid part share information with each other in order to simulate the energy transfer. The best way to ensure it is by moving both the solid and fluid in a separate component, making sure the share topology option is enabled and then running the share prep option.
• A good check to make sure the two parts are sharing topology is to look whether they are forming an interface, which is highlighted by a pink boundary. It is always good practice to check the mesh for any discontinuation of the mesh at the boundaries or by taking a sectional view of the mesh at the interface.

Baseline Mesh:
Baseline mesh is the mesh where no local setting has been done and mesh is completely system generated. The number of elements is 138379. The fluent setting for both the mesh is the same where the k-epsilon standard model is selected and parameters are solved by first order approximation.

Results for above Mesh:

Temperature CONTOUR:

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.

Steamline plot:

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

Velocity at throat:

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.

Temperature at throat:

Heat transfer coefficient plot:

The Wall Heat Transfer Co-efficient is higher at the lower radius throat region.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.

Refined 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 - 150 mm

Body sizing in solid zone with element size - 10 mm

Inflation Layer - No. of layers - 5

Total thickness with maximum thickness - 5 m

Temperature contour:

Steamlines:

Temperature at throat:

It shows that the wall is getting heated because of forced convection, which is predominant at the curvature where velocity is maximum.

Velocity at throat:

The maximum velocity has gone up to 40m/sec, which is 3 points more than previous mesh. It means the refinement is necessary because velocity does have an impact on wall heat transfer coefficient. The velocity at curvature seems to be highest and further downstream it shows a nozzle effect on flow. The contour is more smooth than previous and results are also varying because of finer mesh.

The wall heat transfer coefficient at the wall is captured quite well and profiles seem smoother than the previous mesh. The highest wall HTC value ,which is almost doubled than the previous case value. So, we can safely conclude that these mesh results are in good agreement with actual results and mesh refinement is necessary.

verify if the HTC predictions from the simulations:

• Comparing the obtained simulation data either with the experimental results or with analytical results and the error percentage should be within an acceptable range.
• HTC predictions depend upon theoretical / analytical calulations to begin with, giving us a certain baseline target to acheive . There are numerous correlations for internal, external flows, Compositions to factor in conduction, convection of geometry . Whether it is forced, natural, Flow is laminar, turbulent, different fouling factors for different applications etc

Factors that affect the accuracy of the prediction:

• Mesh size should be finer enough to get accurate values.
• The residual values should befall in the range as per the simulation.
• The geometry of the model should accurate to get accurate values.
• The selection of solver and the boundary conditions should be done carefully as per the conditions.
• The material properties should be given accurately as it represents the real environment.
• The number iterations or time steps should be chosen wisely it should converge the simulation results.

Conclusion :-

From result we can predict that as we reduce the element size, we will be getting more accurate results as well as solution time will be increased. Reducing mesh size means adding inflation will help us to capture accurate data near the fluid solid interation.

• By the above temperature contour it shows that the maximum temperature on the outer wall is at the outlet region of the model because we have 4 inlet and only one outlet so the velocity at the outlet is more. Temperature is proportional to the average kinetic energy of the molecules of the gas. So when temperature increases fluids velocity increases and vice-versa.
• While comparing temperature contour between the base mesh and refined mesh we observed that the temperture will uniformaly distributed in refined.
• The veloicty is greater in the refiened mesh compare to base mesh because in refined mesh we provide a inflation layer so it capture more accurate velocity at the sharp edge.
• The Wall heat transfer coefficient is also greater in the refiened mesh compare to base mesh because the same that we provide a inflation layer in refined mesh so it capture more accurate HTC near the wall.
• So, by the above all comparision we conclude that if we create mesh properly around the model then we definitely got much accurate result.