Conjugate heat transfer Analysis of Exhaust manifold

Tilte:Conjugate heat transfer Analysis on Exhaust manifold

Objective

1. To simulate the exhaust gas (air) in exhaust manifold

2. To find out area from where maximum heat is transfer

3.To find out area where maximum velocity is present

4.Effect of additon inflation layer of wall surface on heat transfer coeff.

5. Effect of increase of body size element on velocity ,temprautre & heat transfer coeff.

6. Effect of coarse mesh on result

7. Effect of Finer mesh on Result

Theory CHT (conjugate heat transfer)

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. 

         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. Forced convection is the most common way to achieve high heat transfer rate. In some applications, the performances are further improved by combining convection with phase change (for example liquid water to vapor phase change).

          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.

Application of CHT

1.Efficiently combining heat transfer in fluids and solids is the key to designing effective coolers, heaters, or heat exchangers.

The fluid usually plays the role of energy carrier on large distances. Forced convection is the most common way to achieve high heat transfer rate. In some applications,the performances are further improved by combining convection with phase change (for example liquid water to vapor phase change).Even so, solids are also needed, in particular to separate fluids in a heat exchanger so that fluids exchange energy without being mixed.

2.Energy Savings

Heat transfer in fluids and solids can also be combined to minimize heat losses in various devices. Because most gases (especially at low pressure) have small thermal conductivities, they can be used as thermal insulators… provided they are not in motion. In many situations, gas is preferred to other material due to its low weight.

3.The forced convection regime corresponds to configurations where the flow is driven by external phenomena like wind or devices like fans, pumps

Following some input is given for the baseline & fine mesh

Case1: Baseline Mesh 

in basline mesh where there is no add any inflation layer & body sizing. the simulation run for 150 iteration

Mesh

mesh generated for basline mesh, where element size is 150mm

Temprature contour

from above plot we can see that higher heat transfer at throat region because there is higher velocity in this area

Velocity Streamline

velocity is minimum at the inlet of pipe but at the outlet bend of pipe there is higher velocity, which result in  higher heat generation

Wall heat Transfer Coefficient

 you can see there is red zone at the bend of pipe where there is more heat is generted , due to higher velocity

Velocity plot

velocity is maximum at the bend of pipe which affect on generation of more heat & also we can see that plot is not smooth

Temprature plot

Case II:Finer Mesh

In fine mesh where there is add inflation layer & body sizing. which result in smmoth flow of air. the simulation run for 150 iteration 

Temprature contour

from above plot we can see that higher heat transfer at throat region because there is higher velocity in this area

Velocity Streamline

Wall heat Transfer Coefficient with inflation layer

you can see there is red zone at the bend of pipe where there is add inflation layers which result in smooth representation of heat transfer. 

Wall heat Transfer Coefficient

 you can see there is red zone at the bend of pipe where there is add inflation layers which result in smooth representation , heat transfer coeff. is avialablr for the first inlfation layer.

Temprature plot

Velocity plot

velocity is maximum at the bend of pipe which affect on generation of more heat & also we can see that plot is smooth.

Effect of Refine mesh on result

             Refine mesh is done by addition of inflation layer & body sizing mesh size give smooth representation of velocity , temprature plot. Also it is found that at first inlfation layer there is smooth representation of heat transfer. Refine mesh gives accurate area where there is higher velocity &temprature.

Verificaion of Heat transfer coeffcient from Prediction 

It is the ratio of heat transferred by convection to the heat transferred by conduction. it shows how much is the heat is transfered due to fluid motion as compared to the heat transferd by fluid by the process of conduction .Always do remember the formula

                                 Nu=hL/K ,         which we get from Nusselt Number,

h= Heat transfer coeff.

L=characteristics length

K is the conductivity of fluid. 

We can also understand Nusselt Number like it is the ratio of conduction resistance offered by fluid if it were stable to the convection resistance offered by fluid.

Therefore, the Nusselt number represents the enhancement of heat transfer through a fluid layer as a result of convection relative to conduction across the same fluid layer. A Nusselt number of Nu=1 for a fluid layer represents heat transfer across the layer by pure conduction. The larger the Nusselt number, the more effective the convection. A larger Nusselt number corresponds to more effective convection, with turbulent flow typically in the 100–1000 range. For turbulent flow, the Nusselt number is usually a function of the Reynold Number and the prandalt Number.

Accuracy of prediction depends on

1 finer mesh give aacurate result & smooth representation of plot

2.Addition of inflation layer arround the waall, give accurate heat transfer coeff. near the wall

3. Accurate conformal mesh at circular section , where fluid is interact with wall surface, which give accurate result.

4.Reduction of body sizing mesh size gives good result &smooth representation of fluid.