Combustion chamber height

We have stated that the combustion of wood occurs in two distinct phases. This understanding has led to the concept of the division of combustion space into two parts. The lower part holds the fuelbed and the upper part provides space that is necessary for the complete combustion of volatiles. These are designated by h_c,f and h_c,c respectively. The sum of these two heights is the distance between the grate and the pan bottom.

We present extensive results on the calculation of these two quantities in the section (v) mentioned above , according to the present understanding of combustion of wood. Thus we shall satisfy ourselves with the reproduction of the required formulae.

(1)

Where is the so called bulk density of the packed bed (see performance). The formula has been written for in g, g/cm³ and A^g in cm² It is obvious that many of these quantities are based on guess work rather than on solid theoretical or experimental data base. Thus as experience gathers in designing the information has to be carefully documented so that one does not go on reinventing the wheel as it were.

The height of the combustion space is determined by two requirements. The first is the question of efficiency and the second concerns the quality of combustion. Since nothing better is known, we will fall back on the flame heights calculated for the open fires with the constant in the expression evaluated through experiments (see flame height). The recommended expression is The recommended expression is

h_c,c = C₂ P^1.4 (2)

C₂ = 75 mm kW^1.4 for fires without grate

= 110 mm kW^1.4 for fires with grate

This height can be checked with the results on the behaviour of the shielded fire with respect to the height of the combustion chamber. According to the discussion there the combustion chamber height should be nearly equal to the grate diameter.

According to Bussmann (1988) this height for an uninsulated stove body may result in significant efficiency losses. Thus he recommends a value of c₂ of 37 mm/kW^1.4

Bussmann's work was of course exclusively concentrated on uninsulated metal stoves. Thus his recipe cannot be expected to be valid for clay/ceramic stoves. Moreover, as was indicated in the discussion about the choice of materials of construction (Table 10.2) the preferred approach should be towards systems with liners. Thus the suggestion here is to stick with the expressions (10.9) and then check the result against three other pieces of information.

The first one comes from Verhaart (1981). His recommendation is to provide for a combustion volume of 0.6 l/kW.

This needs a careful interpretation. Really the power in principle should correspond to the volatile burning power with corrections for unburnt carbon monoxide from the char (see the page on stove performance). The second is to compare with the current practice as summarized in Table 1 and their efficiency and combustion quality which have been presented in the pages on performance and indoor air quality. The final step is to build a prototype and test it along the lines suggested in the above mentioned sections. The procedures suggested above will not only help in developing a product with an assured performance but also will contribute to the development of a sound data base for future design work. However before embarking upon the task of building the prototype few more design aspects need to be taken care of.

Again the presentation is biased towards the single pan stove. The multi-pan stove requires certain changes. For the first kind, indicated in the discussion on power output calculation, the design method for a single pan is applicable. For the second kind, which is the preferred type, combustion should be assumed to continue to occur even under the second pan. Thus the flame length and the combustion volume calculations should correspond to a minimum of half the region under the second pan. Barring fresh modeling work this is the best one can do.

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