Conjugate heat transfer CHT analysis on I C engine inline 4-cylinder engine exhaust manifold by using Ansys Fluent

Aim: Conjugate heat transfer(CHT) analysis on I.C. engine inline 4-cylinder engine exhaust manifold by using Ansys Fluent.

Introduction:

The term conjugate heat transfer (CHT) is used to describe processes which involve variations of temperature within solids and fluids, due to thermal interaction between the solids and fluids. The exchange of thermal energy between the two physical bodies is called study of Heat Transfer, the rate of transferred heat is directly proportional to the temperature difference between the bodies.

`Q = h*A*deltaT`

A typical example is the heating or cooling of a solid object by the flow of air in which it is immersed and some other example includes conduction through solids, free and forced convection in the gases/fluids and thermal radiation.

Conjugate heat transfer corresponds with the combination of heat transfer in solids and heat transfer in fluids. In solids, conduction often dominates whereas in fluids, convection usually dominates. Efficiently combining heat transfer in fluids and solids is the key to designing effective coolers, heaters, or heat exchangers.

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.

CHT can be performed to improve cooling performance of the water jacket and increase engine life. Advancements in cooling for applications such as gas turbines components require improved understanding of the complex heat transfer mechanisms and the interactions between those mechanisms, which our engineers can perform without hassle. Critical cooling applications often rely on multiple thermal protection techniques, including internal cooling, external film cooling, etc. which are efficiently used by our analysis to cool components and limit the use of coolant. We do extensive support for Motor and Battery CHT Analysis to get optimized design for High voltage systems. Conjugate Heat Transfer analysis provides the temperature distribution in solid and coolant of the engine and clear insight on velocity distribution and mechanism of heat transfer of coolant. Results of CHT analysis become input to structural simulations as thermal loads.

Applications: We have a wide experience in Engine CHT Analysis for various type of engines like Bikes, Passenger Cars, Racing cars, Commercial Vehicles, Marine, Agricultural equipment and Earth-moving equipment etc.

The main aim of this study to perform a CHT analysis on a model of an exhaust manifold at different mesh sizes and calculating the velocity, temperature and heat transfer coefficient in both cases.

Methodology:

To achieve the objective of this study following steps are used;

Step 1: pre-processing:

• Importing of geometry: In this step the geometry is imported directly in space claim and creating the fluid volume inside the exhaust manifold.\\

Figure 1: Geometry of exhaust manifold

• Meshing:

Here we performed two types of simulation. First is Base line mesh simulation and second is refine mesh with inflation simulation. In this case we used ‘Shared topology’ method to generate the conformal mesh between solid and fluid surfaces.

Case 1: Base line mesh:

The methodology used for meshing is automatic method with element size of 0.15m. After generating mesh 130073 elements and 26806 nodes are formed. In mesh if more elements is generated smoothly, results are to more accurate from previous simulations.

Figure 2: Meshed structure of Exhaust port Figure 3: Mesh metrics plot between number of element and element metrics

The mesh metric plot of element quality is shown in Figure 3. It identified the elements as per their quality. From figure 4, we can say that elements have their high quality is shown in the range of 0.63 - 1.0 and elements of lower quality is very less. Hence mesh is acceptable range.

Name creation for boundaries:

• Inlet --- top four port,
• Outlet --- at bottom port,
• Outer wall convection --- surface of solid body

Step 2: setup

1. Setting up physics

Solver –steady state, pressure based used.

Model--- in this study we used K-ε turbulent model for set up.

Material properties---

Air: Density=1 kg/m³ and Dynamic viscosity= 0.02 kg-m/s, is used for fluid volume.

Aluminium: is used as solid body

Boundary condition---inlet velocity= 5m/s and temperature = 700K, outlet = 1 atm pressure, Outer wall convection heat transfer coefficient = 20 w/m-k at temperature = 300K.

1. Setting up solution

Hybrid initialization is used for this study.

Step 3: Post- processing

In post-processing we saw the simulated results of plots, contours, and animations. In post-processsing we can also create the locations at which results are simulated.

Results: The generated outputs of case 1 are shown below…

(a) Residuals plot (b) Temperature on outer wall convection:

(c) Temperature contour : (d) Velocity on plane 1:

(e) Velocity stream line: (f) Wall heat transfer coefficient:

Case 2: Refine mesh:

To refine the base line mesh we reduced the element size and using inflation layers on fluid volume surface. The element size is 15mm then nodes 145529 and elements 440152 are created.

Figure 4: Meshed structure of Exhaust port Figure 5: Inflation layers on fuid volume

Figure 6: Mesh metrics plot between number of element and element metrics

Results: The generated outputs of case 2 are shown below…

(a) Residuals plot (b) Temperature on outer wall convection:

(c) Temperature contour : (d) Velocity on plane 1:

(e) Velocity stream line: (f) Wall heat transfer coefficient:

Conclusions:

1. The velocity is maximum in case of refine mesh compare to base line mesh. Because due to inflation layers at fluid surface gives more accurate results. And it is shown at bend portion at red colour.
2. Wall heat transfer coefficient value is more accurate in case of refined mesh than base line mesh.
3. If we compare the temperature contour in both cases, it is found that temperature is more uniform distributed in refined mesh case.

To verify the heat transfer coefficients value:

Nusselt number is required to find \'h\' which is convective heat transfer coefficient. The physical interpretation of Nusselt number is the enhancement of heat transfer due to convection over conduction alone. If Nu=1, then, than your fluid is stationary and all heat transfer is by conduction.

From nusselt number, we can calculate the theoretical value of ‘h’. And then we can compare that simulated value of ‘h’ with theoretical value.

To calculate nusselt number following equations are used ….

`Nu = 0.023*(Re)^0.45*(Pr)^n`…………………..............(i)

where n = 0.45 in case of hot fluid

or 0.33 in case of cold fluid

Then we find the value of ‘h’ from following equations…..

`Nu = (h*L)/K`………………………………………………………………(ii)

Factors which the accuracy of the prediction depend on:

Basically two main factors on which accuracy depend upon….

1. Mesh element size and smoothness
2. Shape or curvature of body