HTC and heat flux during fuel wall impingement

rmotwan · July 11, 2021, 14:02

I'm running gasoline direct injection simulations and 15 CAD after the start of injection (early into intake stroke), some fuel impinges on the piston. I took a look at the heat transfer quantities from the transfer.out for piston and noticed that when the fuel first impinges on the piston, HTC and heat flux shoot up to the orders of 2e05 and 1e07. These high values for HTC and heat flux exist throughout the injection period until the impinged fuel evaporates away from the piston.

Could anyone highlight any errors in setting up the case?

How do I make sure that I'm capturing this fuel-wall impingement process properly?

Thanks,
Rahul

Xiao Ren · July 12, 2021, 18:15

Hi Rahul,

Do you have films in your simulation?

You probably want to look at convective flux (FLUX_CONV and HTC_CONV) from transfer.out. In GDI simulations, sometimes there are quite a lot of films which could mess up the convection calculations, since film heat transfer is conductive.

Best,

rmotwan · July 13, 2021, 11:51

Hi Xiao,

Yes, there is a fuel film on the piston boundary. Could you highlight the necessary changes needed to avoid the convection miscalculations? I'm using the spray model provided in the example GDI case with modifications made to the injector and nozzle to match with the hardware of our experimental engine.

And a follow-up question is: Which htc and flux values should I consider for carrying out the piston thermal analysis HTC/FLUX OR HTC_CONV/FLUX_CONV? I would map the gas-side temperature and htc on the piston surface for carrying out the heat conduction thru the piston.

Thanks for your help,
Rahul.

rmotwan · July 13, 2021, 16:30

Sorry, I should've included this in my previous reply but how are the HTC/FLUX and HTC_CONV/FLUX_CONV calculated? Could you point me towards the documentation, if any?

Thanks,
Rahul

Xiao Ren · July 13, 2021, 18:03

Hi Rahul,

I would suggest you use the HTC_CONV.

The heat transfer coefficient = heat flux / delta_T. If there is film, the delta_T might be influenced by the liquid temperature, which could be a tiny dT. It could yield the large HTC values.

Best,
Xiao

July 11, 2021, 14:02	HTC and heat flux during fuel wall impingement	#1
rmotwan New Member Rahul Motwani Join Date: Feb 2021 Location: Greenville, SC Posts: 18 Rep Power: 5	I'm running gasoline direct injection simulations and 15 CAD after the start of injection (early into intake stroke), some fuel impinges on the piston. I took a look at the heat transfer quantities from the transfer.out for piston and noticed that when the fuel first impinges on the piston, HTC and heat flux shoot up to the orders of 2e05 and 1e07. These high values for HTC and heat flux exist throughout the injection period until the impinged fuel evaporates away from the piston. Could anyone highlight any errors in setting up the case? How do I make sure that I'm capturing this fuel-wall impingement process properly? Thanks, Rahul

July 12, 2021, 18:15		#2
Xiao Ren New Member Xiao Join Date: Aug 2020 Posts: 25 Rep Power: 5	Hi Rahul, Do you have films in your simulation? You probably want to look at convective flux (FLUX_CONV and HTC_CONV) from transfer.out. In GDI simulations, sometimes there are quite a lot of films which could mess up the convection calculations, since film heat transfer is conductive. Best, __________________ Xiao Ren, PhD Senior Research Engineer, Applications Convergent Science

July 13, 2021, 18:03		#5
Xiao Ren New Member Xiao Join Date: Aug 2020 Posts: 25 Rep Power: 5	Hi Rahul, I would suggest you use the HTC_CONV. The heat transfer coefficient = heat flux / delta_T. If there is film, the delta_T might be influenced by the liquid temperature, which could be a tiny dT. It could yield the large HTC values. Best, Xiao __________________ Xiao Ren, PhD Senior Research Engineer, Applications Convergent Science

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July 13, 2021, 11:51		#3
rmotwan New Member Rahul Motwani Join Date: Feb 2021 Location: Greenville, SC Posts: 18 Rep Power: 5	Hi Xiao, Yes, there is a fuel film on the piston boundary. Could you highlight the necessary changes needed to avoid the convection miscalculations? I'm using the spray model provided in the example GDI case with modifications made to the injector and nozzle to match with the hardware of our experimental engine. And a follow-up question is: Which htc and flux values should I consider for carrying out the piston thermal analysis HTC/FLUX OR HTC_CONV/FLUX_CONV? I would map the gas-side temperature and htc on the piston surface for carrying out the heat conduction thru the piston. Thanks for your help, Rahul.

July 13, 2021, 16:30		#4
rmotwan New Member Rahul Motwani Join Date: Feb 2021 Location: Greenville, SC Posts: 18 Rep Power: 5	Sorry, I should've included this in my previous reply but how are the HTC/FLUX and HTC_CONV/FLUX_CONV calculated? Could you point me towards the documentation, if any? Thanks, Rahul