Computer-Assisted Design

In the field of thermal design engineering, so many variables can affect overall efficiency that an optimum design is often difficult to develop. Science has advanced to where we can now precisely predict the results of changing any one variable. But it remains for technology, specifically computers, to be able to predict accurately the net effect of changing many inter-active variables. The exclusive JUCA design was developed with considerable assistance from computers.

The computer simulations begin with specifying the physical size of the fire, the amount of excess air available (the draft setting) and the fire's intensity, which is related to whether kindling or full logs are used, seasoned or green wood, etc. Then for each square inch of the fire's surface, the computer calculates how much radiation hits each square inch of the JUCA's inside walls and heat exchangers.

When this is completed, the computer knows how much total radiation hits each bit of wall. Next, it analyzes the fire and air supply to determine the amount and temperature of the smoke produced. Knowing these results and some aerodynamic information about the heat exchangers, analysis gives the probable smoke velocities and temperatures in all parts of the firebox. Using the Reynolds number, turbulence can be determined. It turns out that a limited amount of turbulence is desirable.

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Next, the computer considers each square inch of the firebox walls and heat exchanger surface. It uses the calculated incoming radiation plus the heat calculated in the smoke right against it, the heat transfer coefficient of the wall, smoke turbulence effects, wall thickness, calculated air temperature and air velocity on the other side of the heat exchanger wall to determine the net heat transfer through that square inch. Then it does the same for every other square inch.

Now that it has calculated all the heat exchangers, it seems that it would be done, but it's not. Since every bit of heat removed from the smoke reduces the available smoke heat in later (upper) parts of the unit and since every bit of heat transferred to the warm air warms it up, much of the heat exchanger system figures to work slightly differently. Therefore, the computer takes all of its results and uses that as its own input to refine the results. Often, it does this many, many times. It is only satisfied when the results for EVERY square inch show a metal temperature within 1°F of its previous results.

Then it gives a printout of the total heat exchange, radiation and convection, and average metal and smoke temperatures for each portion of the units and for the whole unit. Several efficiency figures are also printed along with total output and percent contributions of the various portions. We have run many hundreds of these simulations with different fire sizes, draft settings, and with different construction features in our various products.

Using the computer we can see if a modification to a baffle or heat exchanger will improve or degrade performance before building prototypes. Some interesting results of these computer simulation; for moderate sized fires the stove walls should slope in a 17 degree angle; for large fires, the angle approaches 15 degrees and for small fires, 19 1/2 degrees.

The simulation which follows is a fairly representative one; it took 9 1/2 minutes to calculate on a high-speed computer, shows the design performance of a JUCA B-3B, giving an overall net efficiency of 77% while giving full house heating output with air warmed to 136°F, with JUCA's standard 465cfm blower.

A Sample Computer Simulation

INPUT PARAMETERS
Model B-3B Configuration
Using 8 lb/hr wood
465 cfm blower
100% excess air
Flame temp is 2500°F

CALCULATED QUANTITIES
cfm used by fire = 28.8
effective area on fire 50.6412 sq in
Heat in smoke is 41,990.4 Btu/hr
Radiation absorbed by other wood is 6139.73 Btu/hr
Radiation escaping fire is 9,209.6 Btu/hr
Average Smoke velocity is 0.586653 ft/sec

HEAT EXCHANGER QUANTITIES (Btu/hr)
Area Conv In Rad In Conv Out Rad Out Metal Temp Energy In Smoke Air Temp Smoke Temp
Low Sides 0 2535 1043 1490 214°F --- 65°F 1285°F
Low Front 0 1213 500 712 200°F --- 65°F 1285°F
Low Back 1872 2559 3332 1099 249°F 41990 78°F 1231°F
Low Exch 19340 2397 17864 3872 301°F 40118 118°F 671°F
Mid Exch 6424 506 6150 779 189°F 20779 131°F 485°F
Top Exch 2674 0 2028 647 201°F 14355 136°F 408°F
TOTALS 30310 9210 30917 8599 . 11681 . .

HEAT OUTPUT TO HOUSE (Btu/hr)
AREA CONV OUT RAD OUT AV TEMP % CONTRIB
SIDES 8145 2975 222°F 28.14%
FRONT 2766 1186 215°F 10.06%
BACK 5599 1573 240°F 18.15%
LOW TUBES 10538 2284 301°F 32.45%
UPPER TUBES 3396 430 189°F 9.68%
TOP 474 151 201°F 1.58%
TOTALS 30917 8599 . .

Heat Accounting
Total Energy From Fire 51,200.0 Btu/Hr
Wasted Heat 11,680.8 Btu/Hr
Heat To The House 39,519.2 Btu/Hr

OVERALL EFFICIENCY = 77.18%

Statistical Uncertainty Is -.0062 %

OUTPUT TO HOUSE (Btu/Hr)
35,092.40 WARM AIR AT 136°F AIR TEMPERATURE
2,221.61 UPPER RADIATION AT 136°F SHELL TEMP
39,516.04 TOTAL Usable Output

Heat is given off 88% as warm air and 12% as radiation.

Overall Efficiency is 77.18%

Average Btu/hr/sq ft heat exchanger is 1066
(Maximum Design value is 4,400, so it's operating at about 1/4 capacity)
(This means under these circumstances the B-3B would have capacity of about 160,000 Btu/hr.)

Flue Gas Temperature is 408°F

Various conditions of fire size and blower choice affect the final Overall Efficiency results. In general, they range from 76% to 81% for the Standard blower. The larger optional blowers slightly imporve overall efficiency to 79% to 84% for the various optional blowers.