CHT analysis on Exhaust Manifold

Conjugate Heat Transfer Analysis

Objective: To perform conjugate heat transfer analysis by simulating hot exhaust gas flow through the exhaust manifold placed in free air at room temperature.

Conjugate Heat Transfer:

• 1.Conjugate Heat transfer is the transfer of Heat energy at the interfaces of solid and fluids. A CHT analysis is used to simulate heat transfer from solid body to a neighbouring fluid flow or vice versa. This analysis is generally performed for simulation of Heat exchangers, cooling of electronic devices, general purpose Heating and cooling, HVAC applications, Air/Water cooling applications such as battery packs in EVs etc. In general for CHT, the heat is transferred via convection hence the CHT- coefficient of convective heat transfer, Reynolds number for the fluid flow and therefore velocity of the fluid are crucial parameters.

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.

Analysis setup:

• For this case the CHT analysis is to be performed on an exhaust manifold of an inline 4 cylinder engine, hence has 4 inlets. For simplicity of the analysis the exhaust is flowing simultaneously through all 4 inlets and not in pulses as per the firing order at a velocity of 5m/s. The temperature of the exhaust is 700 Kelvin while the surrounding temperature is at a constant 300 Kelvin. The convective heat transfer coefficient is assumed to be 50 w/m2k.
• The only difference in the 2 cases is the mesh which is crucial for CHT as well as any heat transfer simulation and hence is to be monitored for this challenge.

Case 1- Baseline Mesh:

• The Most basic mesh with default parameters for cell/element size is used to analyse the case. Capture curvature option by default is on and helps with coverage of corners and curved boundaries.

Convergence:

• For a steady state analysis the solution is assumed to be converged when the residuals reach steady state. The same can be said in this case as the residual plots are repeating a pattern with iterations without dropping down in value.

Velocity Plot:

Velocity plot shows the accelerated flow at the corner and overall in the collector pipe of the exhaust where all the exhaust from the 4 exhaust pipes converges into 1, accelerating the flow.

Outer Wall Temperature Distribution:

• As shown in the plot the hot exhaust gases passing through the manifold transfer some of the heat to it via convection. The amount of heat transfer can be related directly with the velocity of the exhaust. The temperature is lower where the flow is stagnate or is moving at the inlet velocity. As the velocity increases, so does the Reynolds number and hence the amount of heat transferred.
• The outside temperature is at 300 Kelvin and the minimum recorded temperature is 366.7 Kelvin which means even at lower velocity of the exhaust flow energy transfer happens but at a lower rate.

CHT :

• Further proof can be given with the CHT plot which follows the similar theory.
• The boundary condition at the outer wall is such that the CHT by convection is 50 w/m2k. As per the plot it drops down to about 26 w/m2k where the flow is stagnated and peaks at 103 w/m2k where the flow is accelerated the most. This dictates the rate of heat transfer as stated previously and can be visualised by the plot here.

Case 2 – Refined mesh with Inflation layers:

• A refined mesh with number of cells close to the 0.5 million figure for the student license are utilised. As defined an inflation of 8 layers with the final thickness of 5mm is provided at the fluid – solid interface inside the manifold pipe, as that is where the heat transfer takes place. Face mesh option is used to refine the mesh on the outer surface of the manifold as at the solid boundary the results are to be interpreted.

Convergence:

As the number of cells increases with finer mesh and better coverage the number of iterations to get a converged solution also increases. The repetition of residual plots is seen at around 1400 iteration here as residuals reach steady state.

Velocity Plot:

• The peak figures are similar as seen in the 1st case but a bit lower. Dispersion is low and the high and low velocity zones are better distinguishable here.
• As the temperature distribution is of importance for this analysis it is okay to have a bit coarse mesh for the fluid as long as inflation is provided.

Outer Wall Temperature Distribution:

• With a finer mesh the temperature distribution is more visible with distinguishable boundaries and less dispersion due to smaller size of cells doing a better job of interpolation of results
• The minimum temperature recorded is lower than that of the first case at 354.266 Kelvin which is more accurate and realistic thanks to the better coverage of the interface where the HT takes place.
• The same relation between flow velocity and temperature distribution can be seen here as low temperature is associated with slow velocity or stagnated flow and high temperature is recorded where the flow is accelerated which can be seen to reach the 600 Kelvin mark in the plot.

CHT :

• Compared to the 1st case a bit higher figures are seen here that is more heat is transferred to the manifold. The maximum CHT recorded with well defined boundary is 109.5w/m2k at the corner of the collector pipe where the velocity of fluid and hence the Reynolds number is higher.
• Even where the flow is slower the CHT is about 28 w/m2k which drops down from the initial 50 w/m2k also lowering the amount of heat transferred.
• It can be concluded that a better mesh at the interface gives more accurate results which further can be improved with finer mesh for the fluid.

• 3. As mentioned in the definition the CHT analysis deals with the transfer of Heat Energy at the interface of fluid and solid and hence the mesh at interface is very crucial and needs to be fine with good quality cells/element to get accurate results. The proof for that can be given by the case-2 results in our case where inflation layers were provided at the interface between fluid and solid.
• Regarding the CHT coefficient, for the best results and ensuring accuracy of the simulation it is always preferred to have experimental data, or reference from a trustworthy source such as case studies, research journals etc. where such figures are calculated.
• The internal flow velocity and its characteristics are affected by the Initial velocity, Pressure, the physical features of the container in this case the manifold pipes etc. which dictates the heat transferred hence CHT analysis is essential prior experimental analysis for better prediction life of the designed component.

Drive link to the folder with all the files:

https://drive.google.com/open?id=1po32qwIrlq-AxWPJEFejM0fa5lZifeM_

PPT Report:

https://drive.google.com/open?id=1xsG7EWJBL44Hd6_JsH_dysqtGAckASbl