Eh? KAS exclaims as he wipes hibernation residue from his eyes.
Here are what I believe are the basics of capture. Assume that every point on a pan base (and some of the side area in the case of a wok) is an emitter of plume effluent. Effluent here being mainly grease, oil, and/or water. In the case of gas cooking, combustion products are also rising with the cooking plume, adding to the plume flow and somewhat to upward velocity.
The effluent from this elemental emission point rises and diverges into a conical shape. Think ice cream cone with ice cream filling. The convolution of all these elemental cones yields a cone truncated at the pan base and extending to the hood. The velocity of the effluent "rays' composing the overall plume will be highest at the center dropping off with angle.
The actual cone in free air will cover more angular space than the hood would cover, but the more extreme rays will be low velocity and easily entrained into the overall flow of air to the hood, assuming that the overall flow is sufficient for containment, not covered in this message.
The hood has to directly capture the stronger part of the cone, and the relevant cone angle w.r.t. the vertical can be the better part of 10 degrees.* And this has to be performed for every likely pot or pan to be used on every burner.
So, at a minimum, for a wall hood the hood capture boundary on three sides has to extend past the likely pan edges. With enough flow we can treat light bars and thick side edges as part of the capture area so long as they are outside the plume-divergence-angle-from-vertical boundary of the potential pan array. For a wall hood, the vertical wall surface acts acts as a reflector to the plume rays.
So what does this mean, practically. Assume that the particular range-top has a pan outline of 48 inches by 24 inches (relative to the wall). If the hood capture plane at the base is at 36 inches above the pan bases, then the 6.5-degree angle (see footnote) implies 4.1 inches of plume growth on the three sides. This would require a hood entry area of 56 inches wide by 28 inches front-to-back -- slightly less at 30 inches high.
What saves us from having to go that deep, particularly with side skirts, curtains, or even cabinets, is that with a wall the hood intake air velocity profile is tilted toward the cook, and this makes it possible to entrain some of the plume rays that would escape over the cook's head into the hood flow.
I don't think any of us can do computational fluid dynamics (CFD) ex mente, so we have to go from experience. And that tells us that 24 inches front to back is likely to be sufficient, particularly if the most greasy odorous cooking is done on rear burners.
Now for the bad news. This monologue was directed at conventional hoods with reasonably defined capture boundaries at the base. What happens with an insert, more or less flush with a flat surface as Wolf directs? Well, all rays that diverge away from vertical toward the cook that hit the flat surface will in principle reflect outward due to momentum conservation. The weaker ones will be dragged inward by the air flow, but note that at a boundary, the drop in air velocity tangential to the intake face falls rapidly, within mere inches.
Wolf's deepest wall hood liner is 22-5/8 deep. If you want more plume overlap on the cook's side then the assembly may need to be spaced out from the wall. Flat plate at the back between liner and wall could reflect impinging plume rays downward where they would be entrained by the main flow.
Choosing a higher actual air flow into the insert than my recommended 90 ft/min will help a bit in extending the effective capture area. If this velocity is met a few inches below the entire flat hood "base" then some front-to-back extension will have been achieved.
My hope is that with perspective taken from this message, and in the context of your cooking style, you can weigh the possible construction options such that you feel you have done your best in making the insert work for you. As you can probably sense at this point, there is a performance trade-off with an insert over a standard canopy hood (typically available in larger sizes) unless you have a custom insert fabricated.
Don't forget: Any type of hood that is combustible has to be protected on the side facing the flame.
ZZZZZ
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*Thermal plumes of kitchen appliances: part 2 Cooking mode, Risto Kosonen, Hannu Koskela, Pekka Saarinen, Energy and Buildings, 2006, Issue 10.
For gas cooking, the plume angle at 0.8m was found to be 7.8 degrees. This is the 1/e velocity value using a Gaussian approximation for the plume velocity shape. If containment flow at the capture area base is based on a 50% of peak plume velocity, then we need to correct the angle to be that at 50% of peak plume velocity. For a Gaussian distribution, that factor is 84% yielding about 6.5 degrees.
The actual phenomena that occur may be envisioned by imagining that over the cooktop there is a gradually widening and slowing plume shape interacting with an inversely oriented hood intake velocity shape where successful capture interaction is defined as diverting plume ray momenta such that their vectors end up directed into the hood capture area. Further, the temperature of the plume rays is being affected by the cooler air pulled from the room (hopefully via MUA). Only CFD analysis can deal with this complex interaction. Arm waving around the details limits us to qualitative advice on hood requirements.
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Cupboard facing diningroom
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