Sound Pressure Level Analysis of a Liquid-Fueled Lean Premixed Swirl Burner with Various Quarls
Round 1
Reviewer 1 Report
This paper explored the noise emission characteristics of a liquid-fueled swirl burner under a wide range of design parameters including the swirling number, core angle of the nozzle tip (so called quarl in manuscripts), and equivalent ratio and acquired some insights or rules for the design. Overall this is a very interesting topic and the authors put lots of efforts with the experimental investigations. But this reviewer believes that it will need some more clarifications in the manuscripts before acceptance. 1. The topic mentions it’s discussing lean-premixed burner. LPP burner is rather a good topic to discuss. As a wide range of parameters have been investigated, did the authors check whether this burner always works under the fully pre-vaporized mode? And further in lean-fuel regime? Or evently targeting to lean-premixed regime with the optimization of noise emission aspect? 2. In Figure 3, the definition of \lambda is not given yet. How does it give mathematically or physically? Does the \lambda=1 case mean stoichiometric ratio? 3. In Figure 4, could the authors give some information about how the air mass flow rate changes with time (s)? Notice thermal-acoustic oscillations or maybe noise emissions would be also affected by the wall temperature of the burner and combustion chamber. It is well-known that the noise characteristics changes both with the equivalent ratio and bulk velocity. Both of them are linked to air mass flow rate. Then, what’s the physics insight of this section 3.1 to present? 4. How does the author change the swirling number in the experiments? 5. The definition of half cone angle of the nozzle tips (the quarl) in context may be not consistent to the Fig. 2. What’s the meaning of the baselined, 0 and 60 degree? It might be more clear to give this details in burner in Fig. 1 instead. 6. The flashback behavior is very important to consider in the design of pre-vaporized and premixed burners, as important as the blow out property. How does this divergent nozzle tip behaves? An convergent-divergent nozzle would be very interesting to further improve the performance.Author Response
Dear Reviewer,
Please, see our responses in the attached PDF file.
Best regards,
Viktor Józsa
Author Response File: 
Author Response.pdf
Reviewer 2 Report
please see attached.
Comments for author File: Comments.pdf
Author Response
Dear Reviewer,
Please, see our responses in the attached PDF file.
Best regards,
Viktor Józsa
Author Response File: Author Response.pdf
Reviewer 3 Report
SOUND PRESSURE LEVEL ANALYSIS OF A LIQUID-FUELED LEAN PREMIXED SWIRL BURNER WITH VARIOUS QUARLS
The authors studied in this work the evaluation of the overall sound pressure level (OASPL) variation of a 15 kW liquid-fueled turbulent atmospheric swirl burner at various setups or operating conditions. Firstly, the combustion airflow rate was adjusted, which induced a swirl number modification due to the fixed swirl vanes. Secondly, the atomizing pressure of the plain-jet air blast atomizer was modified and that affected the swirl number. High atomizing air jets notably increased the combustion noise through the intensifying the shear layer. Thirdly, a modification was performed; from 0° to 60°, half cone angle quarrels in 15° steps were installed on the lip of the baseline burner for extended flame stability. By filtering the OASPL to the V-shaped flames, a linearly decreasing trend was observed as a function of swirl number. The slope of those also has a linearly decreasing characteristic as a function of the atomizing pressure. The conclusion is very interesting and the results gives full knowledge to reduce noise emission. Besides, author talked about a significant reduction in the emission of pollutants. However, he has not show any results obtained with this approach; maybe not evident!
The current work has acceptable industrial potential and academic values. Therefore, I would suggest the acceptance of the manuscript, with specific comments listed below.
Did authors project to use this approach for others atomizing Swirl burners? Why author did not used a CFD codes for approach validation I recommend the author for a further improvement of the burner basic investigations is required out to determine the determining gas concentrations of CO, 02, C02 NOx and SOx,
Author Response
Dear Reviewer,
Thank you for your though work and the valuable comments which are answered below.
Did the authors project to use this approach for other atomizing Swirl burners?The downside of this burner is that the equivalence ratio and the swirl number are closely related to each other, like in many practical turbulent swirl burners. To overcome this issue, a new test rig was constructed: https://crg.energia.bme.hu/rig/ The generalization is expected after a series of specific systematic tests on this burner which has a few axial swirlers and double air inlet to enable precise swirl control or maintaining the swirl number at constant while adjusting the atomization characteristics.
Why did the authors not use a CFD code for approach validation?We have performed a few preliminary analyses, however, it takes time to simulate this burner in detail. For instance, the default evaporation model of ANSYS leads to biased results, and the default thermochemical PDF does not allow the tracking of CH*. Without proper validation, we have concluded that the CFD results would be better to be published later.
I recommend the authors for further improvement of the burner basic investigations are required out to determine the determining gas concentrations of CO, O2, CO2 NOx and SOx.The following update was made regarding the pollutant emission analysis:
‘The pollutant emission analysis is discussed in a previous paper [13]’.
Note that the diesel oil was sulfur-free, hence, the SOx emission was zero. Nevertheless, the used Testo 350 gas analyzer featured this module, but the result was zero at all times.
Reviewer 4 Report
Review of Draft paper 621199 MDPI- Acoustics:
Novotni, G.I., Josza, V.: Sound pressure level analysis of a liquid-fueled lean premixed swirl burner with various quarls.
Synopsis:
The paper presents measurements of overall sound pressure level (OASPL) on a freely burning liquid diesel-fuel swirl flame generated with a tube burner which is shown as a cross section. For liquid fuel atomization a coaxial air-assisted plain jet atomizer is used which is run with varying flowrates. The Sauter Mean diameters of the atomizer were previously characterized and are given in a table. The dimensions of the fuel pipe and the coaxial air-assist nozzle as well as the diameter and length of the mixing tube are given. The combustion air enters the mixing tube through 4 radial holes and 15 45°-angled slots of unspecified dimension. The atomizer is located coaxially in a contoured wall at about the location of the four radial holes followed by the tangential slots. Given the axial momentum flux from the air-assist mass flow of the atomizer the overall swirl number of the burner varies with the mass-flow rates of the main combustion air as well as the air-assist mass flow. The fuel mass flow rate is constant and corresponds to 15kW power. The combustion air is preheated to 400°C whereas the atomization and the fuel are supplied at ambient conditions. With the geometry and flow rates some preheating of these flows can be expected. During a measurement campaign the combustion air flow rate is increased from a minimum of 11.9kg/h in steps of 2.38kg/h until lean blowout occurs. For each step the achievement of steady state of the current operating point was observed before acoustic measurements were taken. These were taken with a sound analyser having a condenser microphone placed at 1m radius with respect to the burner axis using 12kHz sampling rate and 30sec sampling time. The post processing is specified. Using an analysis which cannot be followed, the OASPL results for the operating points are presented in terms of air-to-fuel equivalence ratio and swirl number for the case without quarl and cases with quarls and conclusions are drawn.
Recommendation
The paper suffers from serious procedural shortcomings of the experiments which are outlined below in the comments. Structually it could be significantly improved and adding all necessary information for the reader to analyze the setup.
I recommend to reject the paper.
Comments
While a lot of irrelevant literature with respect to combustion noise has been cited the authors have overlooked a good deal of the previous work in the area. Here is a good review which might help to close that gap.
Candel, S. et al. (2009), Flame dynamics and combustion noise: progress and challenges, aeroacoustics volume 8 · number 1 · 2009
A nomenclature must be added stating all variables used.
All constants used like surface tension, lower heating value, viscosity etc. should be stated.
At 15kW the fuel mass flow rate is about md_fu=0.35g/s (Hu=42000kJ/kg). The stoichiometric air ratio is 14.53kg_air,st/kg_fu. At e.g. phi=0.5 the air mass flow rate is md_air_c=10g/s. Using the ALR given in Table 1 I get the air assist mass flow rate and md_air_s=0.272g/s at pg=0.3bar and md_air_s=0.6g/s at pg=1.6bar. Using isentropic expansion and some heat transfer in the supply duct, I get air assist velocities w_as=240m/s at pg=0.3bar and w_as=356m/s at pg=1.6bar. From measurements taken from the sketch I estimate the angular momentum generated by the combustion air at phi=0.5 and T_air_c=673K to be G_phi=0.002928Nm. G’_x = md_air_c*w_ac + md_air_s*w_as + md_fu*w_fu, so I get G’_x=0.4N at pg=0.3bar and G’_x=0.65N at pg=1.6bar. so the swirlnumber I get is about S’=0.53 at pg=0.3bar and S’=0.4 at pg=1.6bar. I do not get anywhere near the numbers you state in your paper. You must make the calculation transparent.
Table 1 should give the air assist conditions which were assumed / calculated at the nozzle exit, i.e. T_exit, rho_exit, Ma_exit.
Also you must be consistent in using either comma or points for the decimal separator.
Structural improvements: Start the experimental setup with Figure 2, then add the explanations of the burner / atomizer Fig. 1 and finally outline in sufficient detail the procedures yielding the ordering parameters. Then explain the acoustic measurement technique and make sure you state where exactly the microphone was placed as well as how the acoustic environment was.
Serious flaws
The axial location of the microphone was not specified nor were the acoustics of the measurement environment specified. Unless you have an anechoic room measuring sound pressure cannot give an indication of the sound power because you cannot distinguish between propagating and standing waves. As seen from the pictures the flame partly stabilizes inside the mixing tube / quarls, so the sound radiation is changing with the flame geometry. Including quarls the sound radiation pattern will also change, so it is very questionable whether a single point measurement can be generalized to provide a datum of sound emission from the flame.
Operating a free jet flame there is ambient air entrainment which will alter the stoichiometry of the outer shear layer up to the point that significant quenching occurs, in which cases the actual combustion power is less than the nominal which yields the false impression of a quieter combustion when it is just less combustion power.
Author Response
Dear Reviewer,
Please, see our responses in the attached PDF file.
Best regards,
Viktor Józsa
Author Response File: Author Response.pdf
Reviewer 5 Report
This paper describes a sound pressure level analysis of a lean premixed swirl burner with various quarls. Specifically, in this study, is examined the effect of swirl number, the effect atomizing pressure and the influence of quarl. The paper manuscript is well-organized into appropriate sections and well-documented. The "Introduction" contains a detailed review of the literature and it contains the main scope of the submitted study. The "Experimental Setup" is very detailed and all measuring equipment is thoroughly described. All figures in the "Results and Discussion" section are well-shown and thoroughly analyzed. Overall, I cannot find any significant flaw in the submitted study and I surely believe that it should be published as it is since besides its aforementioned assets, it is characterized by high innovation since it analyzes from many aspects i.e. air swirl, atomizing pressure and combustion chamber configuration, a very important issue such as the combustion-induced noise emissions in swirl burner.
Author Response
Dear Reviewer,
Thank you for the positive and supportive review. However, no questions were raised, please, revise the updated version.
Best regards,
Viktor Józsa
Round 2
Reviewer 4 Report
I feel that the authors have only very superficially worked on their manuscript instead of trying to get into the physics. Adding references from Candel's reference list is not reading / understanding them.
Measuring the OASPL does only reflect the amplitude of acoustic pressure at the measurement location. I think you will agree that, if the noise source is inside a room and you measure outside the open door the OASPL is reduced as opposed to measuring inside the room. With increasing swirl the flames form V-flames and retract into the burner. So a good part of the sound emission must first "come out of the door" before it hits the microphone. And what comes out is only a fraction of what it was, when that part of the flame is in the open.
The analysis of OASPL vs S' doesn't make sense, because you change the stoichiometry at the same time. In premixed combustion noise theory a low turbulent Damköhler number has a significant influence on the sound emission which is reduced with low Da-number. But unfortunately in your setup one cannot distinguish between what comes from coherence volume scaling with Da_t and what is just reduced sound emission because of the flame retraction.
What good is the "slope of OASPL"? To me the slope of OASPL means:
d(OASPL)/d(S')
In the V-flame mode that seems essentially constant in the developed V-cases but for the 45/60° quarls. Without proper analysis of what is turbulent combustion noise and what comes from intermittent flow in the unstable quarls there is not much more to see.
There are outright errors which were not corrected. E.g. I remarked that the We number in Table 1 didn't have the proper decimal separator. The authors reply is "..Maybe this reviewer has noticed that of We_A in Table 1 which is correct since the commas here separate the thousands,..."
Now putting the data from Table 1 and 2 into eqn. 3:
We_a = 1.27*207^2*0.4E-3/28E-3=777 =! 511041.
and next putting that value We=777 into eqn(2) together with the AFR from Table 1
SMD=0.66*0.4E-3*777^-0.5 *(1+1/0.778) = 21.6E-6
which is indeed the SMD shown in Table 1.
If I can see right away that the order of magnitude of We_A isn't correct why can't the authors? How should I trust their results at all?
Table 1 shows Ma numbers in excess of 1. For a convergent nozzle at these pressure ratios this will not happen. Fluid mechanics basic course.
This is rather sloppy and should not happen in a Journal article.
So I still think this paper has no archival value and should not be published in a journal.
Author Response
Dear Reviewer,
Please, see our responses and corrections attached.
Author Response File: Author Response.pdf
