Due to the need for energy conservation in buildings and the simultaneous benefit of cost savings, the development of a low firing rate load modulating residential oil burner is very desirable. One of the two main requirements of such a burner is the development of a burner nozzle that is able to maintain the particle size distribution of the fuel spray in the desirable (small) size range for efficient and stable combustion. The other being the ability to vary the air flow rate and air distribution around the fuel nozzle in the burner for optimal combustion at the current fuel firing rate. In this paper, which deals with the first requirement, we show that by using pulse width modulation in the bypass channel of a commercial off-the-shelf bypass nozzle, this objective can be met. Here we present results of spray patterns and particle size distribution for a range of fuel firing rates. The results show that a desirable fuel spray pattern can be maintained over a fuel firing rate turndown ratio (Maximum Fuel Flow Rate/Minimum Fuel Flow Rate) of 3.7. Thus here we successfully demonstrate the ability to electronically vary the fuel firing rate by more than a factor of 3 while simultaneously maintaining good atomization.
Because of modern energy efficient design, the heating requirements for new construction are significantly lower than old residential buildings. Thus a lower firing rate boiler is sufficient to fulfill the peak demand. This is especially true for smaller homes. Secondly, because the heating requirements during a given day continuously change with time due to the changing weather conditions and demand from the occupants, a conventional residential boiler must cycle (on/off) many times during a day. During the starting phase the combustion efficiency of oil burners is much lower than that during the steady state operation; consequently the level of pollutants emitted is higher.
Excessive on/off cycling leads to an overall reduction in the energy efficiency of the heating system and an increase in the environmental pollution contributed by the oil burner. Furthermore, it also results in larger temperature swings within the house. Excessive on/off burner cycling also leads to frequent plugging of the swirl nozzles. A low firing rate load modulating oil burner will not only provide more thermal comfort for the occupants, it will also result in lower fuel costs and a reduction in the pollutant emitted.
Standard pressure swirl nozzles are designed for a constant oil firing rate. If one attempts to lower the oil firing rate by reducing the pump supply pressure, the resulting atomization is poor (larger fuel droplets), thus leading to poor combustion. Bypass pressure swirl nozzles, such as those from Delevan, do allow operation at lower firing rates; but the spray quality (fuel droplet mean size) becomes poor. Therefore the first requirement for the development of a variable firing rate oil burner is a fuel spray nozzle whose fuel firing rate can be turned down (by a factor of two or more) without degrading the fuel droplet mean size.
Krishna et al. [
Kohlmann [
For the same objective of developing a variable load oil burner, Drabo et al. [
Muller et al. [
Here, we are reporting on the results of an alternative approach which is closely related to the method used by Krishna et al. [
The experiments were divided into two parts. The first part consisted of performing droplet size distribution measurements under various operating conditions (PWM duty cycle, PWM frequency, fuel temperature). For these tests we used water as a simulant fuel. Extensive atomization measurements carried out at Brookhaven National Laboratory in the past have shown that there is no significant difference in the droplet size distribution measurements between “water” and “No. 2 heating fuel”. Although a variable fuel firing rate spray nozzle is only one requirement for the load varying oil burner, the other being the controllable varying retention head design for the burner to adjust and modulate the air flow around the spray nozzle, we nevertheless also performed combustion tests in a conventional retention head burner to demonstrate at least a stable flame under various fuel firing rates with the pulse wave modulated bypass and a bypass nozzle.
Droplet Size MeasurementsA schematic of the experimental setup for droplet size distribution measurements is shown in
The solenoid valve discharges the bypassed water back into the water supply tank that is mounted on an electronic scale. A video of a stop watch and the scale front panel are recorded during the particle droplet size measurement period for the purpose of computing the net fuel flow rate through the spray nozzle. For these tests an RC SL4-205 gasoline fuel injector was used as Solenoid Valve for Bypass Flow Control.
A Malvern Spraytec System, which is a laser diffraction based instrument, was used to measure the droplet size distribution within the liquid spray at various liquid temperatures. This system has a particle detection range of 0.1 µm to 900 µm. Additional details for this system are provided in Reference [
The variation of water flow rate through the nozzle as a function of PWM duty cycle at two different water temperatures is shown in
The measured volume averaged mean diameters (D [4, 3]) for the corresponding (
During the droplet size measurement experiments we also recorded videos of the nozzle spray for various duty cycles immediately after recording the Malvern droplet size distribution. A cell phone video camera with macro lens attachment was used for this purpose together with a stroboscope for backlighting the spray. The stroboscope frequency was adjusted so as to match (as closely as possible) the framing rate of the video camera during filming. Stills from these videos are presented in Figures 6-8. A web link for viewing the videos for these conditions is given: https://1drv.ms/f/s!AhL6U6Talgxmkyvk1e579VPHUOis.
From these figures and videos we see that the spray is much finer at higher water temperature. Also, at the lower duty cycle of 40%, the spray is fine. In fact we see that the conical film sheet is breaking up early. We also note from
A variable firing rate spray nozzle is only one requirement for a load modulating oil burner. Clearly, not only must the mass flow rate of air going through the burner be regulated to match the fuel flow rate, the velocity distribution of air going through and around the fuel spray must be changed as well to maintain a stable flame and combustion quality (low emissions of soot and pollutants). Conventional residential oil burners only have very limited controls for adjusting the air flow rate. These provide a crude and limited control for the air inlet area to adjust the air mass flow rate going through the oil burner. As shown in
Nevertheless, we did want to demonstrate and verify that we could at least get a stable flame by using an off-the-shelf bypass nozzle at very low fuel firing rates. Due to logistical and time constraints the combustion tests needed to be performed at a separate location while the droplet size distribution measurements were still in progress. As a result, these combustion tests could not be performed using the pulse wave modulated bypass nozzle. Therefore, for these tests we only used the manual control valve on the bypass line to control the fuel firing rate. Since atomization is expected to be only better with PWM control, these tests are conservative.
A schematic of the experimental setup for flame visualization tests is shown in
We performed droplet size distribution measurements of fuel sprays (with water as the fuel simulant) at various fuel firing rates by controlling the fuel flow rate via pulse width modulated bypass flow from an off-the-shelf bypass nozzle. The results show good atomization properties of the spray over a 3.7 to 1 fuel flow rate ratio. Even though the droplet mean size is lower at higher (minimally flashing) fuel temperature, at lower flow rates the difference is not much. Thus heating of the fuel to high temperature to achieve very low fuel firing rates may not be necessary. Combustion tests with a conventional burner (with crude and un-optimized air flow distribution) also demonstrated a stable flame over a wide range of fuel flow rates. The next logical step towards completing the development of a variable load residential oil burner is to design such an oil burner with dynamically controllable air mass flow rate and velocity profile (in and around the droplet spray). This may require a controllable swirler in front of the spray nozzle and/or retention head. Modified design of the flame tube may also be required. Computational Fluid Dynamics (CFD) simulations should prove to be an important tool for arriving at suitable design options for this.
The authors would like to thank Dr. C. R. Krishna, Yusuf Celebi and Mr. Bennie Mwiinga for their assistance in the laboratory throughout this project. They would also like to thank the Alabama Louis Stokes Alliance for Minorities Participation (ALSAMP) for their support during this project. This project was supported in part by the National Science Foundation-College Research Teams Program (NSF-CRTP) and in part by National Oilheat Research Alliance (NORA).
The authors declare no conflicts of interest regarding the publication of this paper.
Drabo, M.L., Tutu, N.K., Butcher, T., Wei, G. and Trojanowski, R. (2018) Evaluation of a Pulse Width Modulated Bypass Nozzle for the Development of a Variable Load Residential Oil Burner. Engineering, 10, 643-654. https://doi.org/10.4236/eng.2018.1010047