| With our country’s development of economy and increasing of energy consumption,energy saving technology and the use of clean energy sources play an important role,so gas boilers having been widely applied in our lives.Recovering boiler’s exhaust heat can effectively improve energy efficiency and reduce environment pollution. Gas boilers’ smoke ejection temperature is between(140 ~ 230 ℃) and the volume fraction of the water vapor ejected in the flue gas is up to 20%. High-temperature water vapor contains large sensible heat and latent heat, and the energy having considerable value for heating water of daily consumption.Applying condensing heat exchanger, flue gas temperature can be reduced to below the dew point temperature to recycle waste heat.In the paper,condensing heat exchange of gas boiler’s exhaust has been systematically studied.Through researching and analyzing condensing heat exchanger we found that when we apply indirect contact condensing heat exchanger to recycle waste heat,it’very difficult to reduce temperature below the dew point temperature with larger volume of heat exchanger and smaller amount of water vapor condensation.Applying direct contact condensing heat exchanger to condense high-temperature smoke needs lower coast and the utilization of waste heat is higher.Firstly,analyze direct condensing heat exchanger by numerical simulation method based on three-dimensional physical model.We get the temperature distribution, the volume fraction distribution for water vapor, velocity distribution in the flow field.Direct contact condensing heat exchanger can enable heat thoroughly mixed of hot and cold fluid and the temperature of smoke ejection reduces obiviously.Cooling water can absorb a lot of sensible and latent heat from exhaust condensation heat transfer.When it is close to the cold fluid inlet that temperature is lower and vapor condensation is best.When the flue gas flow rate increases, cold and hot fluid mixed time in the exchanger is reduced.When flow rate of flue gas is too fast it can cause condensation is not sufficient.The amount of water vapor condensation curve changes with the flue gas velocity is reduced after the first increase. When the volume fraction of water vapor in the flue gas increases, the water vapor partial pressure increases and condensation effect will be enhanced. The amount of condensation of water vapor directly reflects the efficiency size of waste heat recovery.Secondly,applying different three-dimensional physical model to compare the effect of smoke condensation.Enhancing the entrance position of cold fluid can increase the exchanging time of cold and hot fluild in the heat exchanger,and it can reduce the volume fraction and temperature of water vapor at the exit effectively.It can lessen the flow of cold fluid that the velocity of cold fluid reduces or pipe diameter of entrance diminishes,as a result,it has an influence on condensation and heat exchanging effect of smoke.In different conditions,when we imitate condensation and heat exchanging in horizontal pipe model and staggered pipe model,condensation effect changes with the velocity of cold fluid. When the inlet velocity of cold fluid is 3m/s condensation effect in parallel pipe is better than staggered pipe and when the velocity is 1m/s the result is converse.When high-temperature smoke are going through mixed model to condense and change heat,it’s condensed by cold fluid in coiled pipe first when going by,and it’s condensed one step further by fluid drop,so recover rate of waste heat increases. Condensation effect of flue gas in condensing heat exchanger for metal wall is better than that for adiabatic wall.So the wall does not need to be adiabatic.Finally,to analyze and track the movement of dust particles in heat exchanger by discrete phase model.Since the hot and cold fluid are mixed with each other the flow path of particles is more complex. Particles will be out of the heat exchanger in a short time.Only a few rotating particles stay longer in the heat exchanger. |
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