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Step 8: Postprocessing

  Review the solution by examining graphical displays of the results and performing surface integrations at the combustor exit.

1.   Report the total sensible heat flux.

figure Reports figure figure Fluxes figure Set Up...

figure

(a)   Select Total Sensible Heat Transfer Rate in the Options list.

(b)   Select all the boundaries from the Boundaries selection list.

(c)   Click Compute and close the Flux Reports dialog box.

Note:   The energy balance is good. The net result is small compared to the heat reaction.

2.   Display filled contours of temperature (Figure  14.4).

figure Graphics and Animations figure figure Contours figure Set Up...

(a)   Ensure that Filled is enabled in the Options group box.

(b)   Ensure that Temperature... and Static Temperature are selected in the Contours of drop-down lists.

(c)   Click Display.

Figure 14.4: Contours of Temperature--Variable $C_p$
figure

  The peak temperature has dropped to approximately 2300  ${\rm K}$ as a result of the temperature and composition-dependent specific heat.

3.   Display filled contours of specific heat (Figure  14.5).

figure Graphics and Animations figure figure Contours figure Set Up...

  The contours of the mixture specific heat will show the variation of the specific heat within the domain.

(a)   Select Properties... and Specific Heat (Cp) from the Contours of drop-down lists.

(b)   Click Display and close the Contours dialog box.

  The mixture specific heat is largest where the CH $_4$ is concentrated, near the fuel inlet, and where the temperature and combustion product concentrations are large. The increase in heat capacity, relative to the constant value used before, substantially lowers the peak flame temperature.

Figure 14.5: Contours of Specific Heat
figure

4.   Display velocity vectors (Figure  14.6).

figure Graphics and Animations figure figure Vectors figure Set Up...

figure

(a)   Enter 0.01 for Scale.

(b)   Click the Vector Options... button to open the Vector Options dialog box.

figure

i.   Enable Fixed Length.

  The fixed length option is useful when the vector magnitude varies dramatically. With fixed length vectors, the velocity magnitude is described only by color instead of by both vector length and color.

ii.   Click Apply and close the Vector Options dialog box.

(c)   Click Display and close the Vectors dialog box.

Figure 14.6: Velocity Vectors--Variable $C_p$
figure

5.   Display filled contours of stream function (Figure  14.7).

figure Graphics and Animations figure figure Contours figure Set Up...

(a)   Select Velocity... and Stream Function from the Contours of drop-down lists.

(b)   Click Display.

Figure 14.7: Contours of Stream Function--Variable $C_p$
figure

  The entrainment of air into the high-velocity methane jet is clearly visible in the streamline display.

6.   Display filled contours of mass fraction for CH $_4$ (Figure  14.8).

figure Graphics and Animations figure figure Contours figure Set Up...

(a)   Select Species... and Mass fraction of ch4 from the Contours of drop-down lists.

(b)   Click Display.

Figure 14.8: Contours of CH $_4$ Mass Fraction
figure

7.   In a similar manner, display the contours of mass fraction for the remaining species O $_2$, CO $_2$, and H $_2$O (Figures  14.9, 14.10, and 14.11). Close the Contours dialog box when all of the species have been displayed.

Figure 14.9: Contours of O $_2$ Mass Fraction
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Figure 14.10: Contours of CO $_2$ Mass Fraction
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Figure 14.11: Contours of H $_2$O Mass Fraction
figure

8.   Determine the average exit temperature.

figure Reports figure figure Surface Integrals figure Set Up...

figure

(a)   Select Mass-Weighted Average from the Report Type drop-down list.

(b)   Select Temperature... and Static Temperature from the Field Variable drop-down lists.

  The mass-averaged temperature will be computed as:


\overline{T} = \frac{\int{T \rho {\vec{v}} \cdot d{\vec{A}}}}{\int {\rho {\vec{v}} \cdot d{\vec{A}}}} (14.13-2)

(c)   Select pressure-outlet-9 from the Surfaces selection list, so that the integration is performed over this surface.

(d)   Click Compute.

  The Mass-Weighted Average field will show that the exit temperature is approximately 1834  ${\rm K}$.

9.   Determine the average exit velocity.

figure Reports figure figure Surface Integrals figure Set Up...

figure

(a)   Select Area-Weighted Average from the Report Type drop-down list.

(b)   Select Velocity... and Velocity Magnitude from the Field Variable drop-down lists.

  The area-weighted velocity-magnitude average will be computed as:


\bar{v} = \frac{1}{A} \int{v \: dA} (14.13-3)

(c)   Click Compute.

  The Area-Weighted Average field will show that the exit velocity is approximately 3.29  ${\rm m/s}$.

(d)   Close the Surface Integrals dialog box.

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