| OCR Text |
Show The data in Figure 6 address whether or not the thermal efficiency is compromised by burning lean. This concern stems from the fact that lean flames have lower flame temperatures that can affect heat transfer. Both the 15 and 18 kW (50,000 and 60,000 BtuIh) runs reached thermal efficiencies of 80% for $ > 0.75 with only a slight decrease towards $ < 0.7. Again, a drop-off in efficiency for the 12 kW runs is found. 2300 0.9 2200 0.8 rf> ~ II() o • ~ ~ ~ 0 0.7 •0 • 0 • x /' / 2100g ~ .- .- x .~.. ~ x x ~ = ~ 0.6 = 2000 'E ~ ", .- ~ .- 0.5 ~ /' c.. = x c.t-i .- 1900 ~ ~ 0.4 .- ~ 12kW ~ 0.3 x 1800 ~ ,- • 15 kW / - 0.2 0 18kW ~ _ _ _ _ _ _ Ad. Temp 1700 O.l 0 1600 0.6 0.7 0.8 0.9 Equivalence Ratio Figure 6: Efficiency versus $ for constant power and water flow rates The weak dependency of efficiency on $ can be explained by the fact that energy transferred to the heat exchanger is linearly dependent upon only two variables. The fust variable is the temperature difference between the combustion products and the heat exchanger, with the second variable being the heat transfer rate, h, from the combustion products to the water. Therefore, if h remains constant, a decrease in the flame temperature (i.e. a drop in $) would lead to a linear decrease in efficiency. However, in the present system this decrease is offset by an increase in h. During these tests, <t> is decreased by adding more air to the premixture. This leads to a corresponding increase in 11 |
