A simplified CFD model for radiative heat transfer in engines

Transcription

1 A simplified CFD model for radiative heat transfer in engines Chandan Paul, Daniel C. Haworth The Pennsylvania State University at International Multidimensional Engine Modeling Meeting Detroit, MI April 9 th 2018

2 Outline Radiation modeling in engines using detailed radiation models Results for a VOLVO 13L Diesel Engine using detailed radiation model A simple radiation model for CFD solvers 2 of 14

3 Engine configuration A 13L heavy-duty VOLVO-truck engine (diesel) 60-degree wedge centered on one of six spray-plumes Simulations begin at 60 o btdc (after intake valve closes), and continue until 120 o atdc (before exhaust-valve opening). Engine parameter CR : 15.8 Bore : 131 mm Stroke : 158 mm RPM : 1213 Cases Initial and peak pressure Spray EGR (mass fraction) Full-load bar 200 bar 47 mg (- 4.6 to16.8 CAD) CO 2 (3.92) H 2 O (1.49) O 2 (18.84) N 2 (75.75) Part-load 5.65 bar 85 bar 13 mg (- 2.8 to 3.5 CAD) CO 2 (4.06) H 2 O (1.54) O 2 (18.68) N 2 (75.72) 3 of 14

4 Species conservation Y t Turbulent combustion and radiation modeling uy x j j J Yu j j S, chem S,spray xj xj Energy conservation h hu hu j j t x x x j j j D p u Q Q S Dt i ij rad chem h, spray x j Combustion models: Well-Stirred Reactor (WSR) model No Turbulence chemistry interactions (TCI) Transported PDF (tpdf) Can capture TCI J h j Radiation models: Spectral model -- Gray -- k-distribution -- Line-by-line RTE solvers -- Optically thin (OT) Only emission -- P1 No TRI -- Photon Monte-Carlo (PMC) TRI Radiative Species -- CO 2 -- H 2 O -- CO -- Soot HITEMP2010 HITRAN2008 Rayleigh s theory In order of increasing computational effort and accuracy tpdf + PMC + Line-by-line most accurate 4 of 14

5 Results: Global effect of radiation on heat transfer Full-load Part-load Variable WSR/PMC/LBL (% w.r.t Conv. heat loss) tpdf/pmc/lbl (% w.r.t Conv. Heat loss) Emiss Reabs (89% of emis.) Rad. loss Variable WSR/PMC/LBL (% w.r.t Conv. Heat loss) tpdf/pmc/lbl (% w.r.t Conv. Heat loss) Emiss Reabs (83% of emis) Rad. loss of 14

6 Results: Soot vs gas radiation Full-load engine-out pollutant emission NO (ppm) Soot (mg) Experiment e-3 tpdf/pmc/lbl/tri e-3 Species Emission (J) Absorption (J) Reached wall (J) CO H 2 O CO Soot Radiative loss by: Gas: 98% Soot: 2% Global soot volume fraction: 0.25 ppm, Max. local soot volume fraction: 8 ppm In terms of radiation reaching walls H 2 O radiation dominate. Radiative emission from CO 2 dominate. 6 of 14

7 Results: Soot vs gas radiation (increased soot) Artificially increase soot amount by 100 times Global soot volume fraction: 25 ppm, Max. local soot volume fraction: 800 ppm Radiation heat transfer is 10% of convective heat transfer. Species Emission (J) Absorption (J) Reached wall (J) CO H 2 O CO Soot Radiative loss by: Gas: 53% Soot: 47% In terms of radiation reaching walls, now soot dominate over other species. CO 2 emission still dominate. Radiative heat loss is 10% of convective heat loss. 7 of 14

8 Results: effect of TRI With TRI emission and reabsorption both increases marginally -- Under normal operating condition : 3% increase -- Under increased soot (100 times) condition : 5% increase 8 of 14

9 Results: Spectral analysis Infrared Visible Radiative emission from gas molecules occurs in infrared region. Important wavelengths are 2.7 μm and 4.3 μm. Broadband spectrum for soot emission. In the visible region only soot emits. 9 of 14

10 A simple radiation model (P1/StepwiseGray) for engine Divide the whole spectrum in five bands based on 2.7 μm and 4.3 μm. For each species we can develop following table: P T κǁ 1 b1 b2 b3 b4 b5 κǁ 2 κǁ 3 κǁ 4 b1 b2 b3 b4 b5 T I b1 I b2 I b3 I b4 I b5 κǁ 5 Tabulate Calculate I I d b 1 i, Ibi b Ib d i, cell ( p, T, Y ), ( T) bi cell and using the lookup table. Solve P1-RTE for each of the bands. I d 10 of 14

11 Simple radiation model reabsorption calculation Using new model (P1/StepwiseGray) the error is less than 10% of PMC/LBL, with significant improvement in computational cost. 11 of 14

12 Results: P1/StepwiseGray band analysis P1/StepwiseGray captures the bandwise reabsorption quite accurately. P1/Gray failed to capture band-wise reabsorption. 12 of 14

13 Simple radiation model Contours and computational cost Total time (hr) Rad. Time (hr) WSR/PMC/LBL (100 rays per cell) (90%) WSR/P1/FSK(16 RTE) (80%) WSR/P1/Gray ( 2.5%) WSR/P1/StepwiseGray (22%) Emission and reabsorption contour of P1/StepwiseGray matches PMC/LBL quite closely. Significant saving in computational cost. 13 of 14

14 Summary and concluding remarks VOLVO 13L engine full-load and part-load operating condition. Radiation heat loss is ~5-10% of convective heat loss. Approx % of convective heat loss is redistributed due to radiative reabsorption. Under current engine operating condition gas-radiation is more important than soot radiation. With TRI emission and reabsorption increases marginally (~3-5% of no-tri values) A simple P1/StepwiseGray model for CFD solvers. Thank you. Thank you! 14 of 14