Downdraft Vent Hood Flow Analysis

Downdraft Vent Hood Flow Analysis

Downdraft Range Vent Hood Flow Analysis

One of the cool kitchen features that caught our attention when we saw what would become our house was that the stove had a retractable range vent.  It would pop up behind the stove, and sucks the air away from the side, then down through the counter, down through the floor, and out the side of the wall.  What really makes these things great is they open up the whole kitchen, as we have one big kitchen/dining room/living room.  A traditional stove hood would have been right in the middle of the room, and would have covered up our large patio windows.

 Now, of course after we moved in and started using it, I was less than impressed.  The range has the largest gas burners at the front, so those are the ones we use the most for boiling pasta water or cooking on a skillet.  Steam is great to see air flow, as it’s a white color, and I could see it wasn’t making its way into the vent (see link below for example downdraft vent with steam from pot).  So of course, this was one of the household applications I wanted to analyze with FloEFD, to see what was going on.

 http://homeinteriornews.com/wp-content/uploads/2012/01/3504f_Home_Appliances_downdraft-range-hoods-sirius-sdd4.jpg 

First, I looked up our downdraft vent brand, and found the user/installation manual.  It mentioned the blower was was rated to 600 CFM.  Perfect, if I ignored filter and duct loses, I had everything I needed to model this ventilation system.  Of course, being CAD lazy, I needed to find a model of a cook top.  Luckily, I found one on grabcad.com.  After building a pot of water to include in the analysis, things didn’t quite look right, the pot seemed to be too large for the stove.  Seems the free stove model I got was from somewhere with smaller kitchens then here in California.  I decided to press on and see what the results would tell me.  Maybe downdraft vents are designed more for use in smaller kitchens?  FloEFD would tell the story.

Downdraft Ventilation FloEFD model with Pot of Boiling Water

The last bit of info I needed for my analysis was the flow rate of the steam from the boiling pot, as I wasn’t going to actually analyze that process.  As with any engineer, I have a boss who wants me to get my work done as fast as possible, and I need my computer to run simulations as fast as possible so I can have it simulate something else.  It was much faster to provide a flow rate for steam.  Looking into it, according to the internet, a gallon of water will create 1700 cubic feet of steam.  According to my box of Mac and Cheese, I should boil 6 cups of water which is roughly a half a gallon.  From experience, I know that amount will boil off in about 2 hours, give or take.  So we have a steam flow rate of about 7 CFM, higher than you’d think right?

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Now the fun part, let’s look at the results.  I could have put a bunch of pots on the stove, as I’m sure the performance of the vent would be lower if our steam had obstacles in its way.  But, I rarely have a bunch of things on the stove at once, so I thought that was a little unfair to the downdraft vent.  I also thought of including the carbon dioxide and monoxide coming from the burner, which is very important as it’s the real reason we have the ventilation in the first place.  But I believed that as the pot is already so high as to be in line with the vent, it would be the tougher task to suck into the vent.  The exhaust gases from the burner are lower, so have more time to be pulled into the vent as they rise. 

Below are some results looking at the steam concentration (isosurfaces and flow trajectories).  You can see from the isosurfaces all the steam is going into the vent.  I think using spaghetti shaped flow lines made sense for my boiling pasta water pot    You can also see though, that a fair amount of the flow that makes it into the downdraft vent comes from behind the vent, not infront of it.  So I believe this is a design issue unique to the downdraft vent.  I don’t think traditional range hoods have an issue of sucking in air from near the ceiling, it’s all coming upward from the stove.  Plus the hot gases/steam are already rising up because of bouyancy.  Maybe some extendable side ducting fins would help the vent suck in 600 CFM from the front of the vent, the important stove area, instead of just the surroundings.

Isosurface of Steam at concentration of 10,000 parts per million (volume fraction of 0.01)

Isosurface of Steam at 1000 parts per million (Volume fraction of 0.001)

Streamlines from pot of boiling water

Flow Lines going into the Downdraft Vent

 It seems like my vent is doing a perfect job.  Now we get to something I’ve done a thousand times working on support cases.  My simulation results don’t match my “experimental” results (i.e. cooking observations).  This is where my analysis stalled for a bit while I thought long and hard for that one key fact that I was missing.  It’s always a lot easier to find someone elses mistake then it is to find your own…

What I realized is that we never use the downdraft vent on high power.  It sounds like a freight train, and completely drowns out any conversation which is the whole point of the open concept room, to allow you to cook in the kitchen and still talk with people in the living room at the same time.  I always use the medium setting because while it’s still not whisper quiet, it can be talked over.

I had no idea what flow rate the medium setting would provide, so I set the flow rate to 300 CFM, half of the 600 CFM high setting.  That felt like medium to me.  Here are the FloEFD results.

Isosurface of steam concentration of 100,000 ppm at Medium airflow rate

Isosurface of Steam concentration of 10,000 ppm at Medium Vent flowrate

Isosurface of Steam concentration of 1,000 ppm at Medium Vent flowrate

Flow Lines from boiling pot

Well now the experiment and simulation results are matching, which is always a good feeling.  It seems like on medium setting, the vent is doing basically nothing to the steam leaving the pot.  This was exactly what I was seeing when I was cooking, but I wasn’t sure if the vent was at least diluting the steam by inducing fresh air into the cooktop area.  During my research for this, it seems like other models of downdraft vents have higher CFM’s.  Maybe my vent was a a low end unit, and the blower is underpowered for this application?   Maybe, but I think if the vent was able to better direct where it was getting the air from, the important stove air, instead of getting air from behind the vent, then even on the medium setting it would likely perform a lot better.  I also think though if the unit was quieter, it would be a lot better.  To have 600 CFM flowing through the narrow thickness of this vent, it’s going to be loud unless analysis is done to minimize these effects.  Maybe that’s an analysis for another day.  In the meantime, I’ll simply crank the volume on my TV when I cook so I can use the maximum setting on the vent. 

 Next blog topic is close to my heart.  Analyzing my propane grill with my smoker box to fine hotspots/cold spots, and smoke distribution within the grilling area.

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