Preliminary Analysis of the Performance of an Electric Supersonic Propeller

Round 1

Reviewer 1 Report

 

1. Why in figure 2 the station are numbered 0,1,2 and then 5 and 9?

2.  what is C0 in equation 1? 

3. at the wall temperature condition why convective heat transfer is neglected?

4. It is not clear why the equation 7 has to be solved iteratively? how is deltac calculated?

5. what does each term in table 3 means ? how there are used in the calculated of power efficiency?

 

The quality of english has to be improved a lot in the submitted manuscript for example

1. line 3, "and further and" is wrong

2. line 5 , " so too far," is wrong

3. line 20, "higher energy density and higher power density," is repeated

Like the above-stated examples, there are several places where the quality of English has to be improved. Kindly note that several places selling mistakes are there.

 

Author Response

Dear Reviwere 1,
Thank you for your comments, please see my responses below.

1. As mentioned in the text in line 48:
   The engine station numbering is chosen to be consistent with common jet engine  notation, where station 0 indicates freestream quantities, station 2 is just upstream of the compressor and station 5 is just behind the turbine. Stations 3 and 4 are missing because they are in between the compressor and the combustor and between the combustor and the turbine respectively, stations 6, 7 and 8 refer to afterburners and internal nozzles, neither of these exist in this propulsion system and are, therefore, omitted. Omitting numbers for stations that don't exist in a propulsion system is common practice in jet engine station numbering. So, in order to be consistent with jet engine notation, the presented numbering is chosen.
2. c0 is the absolute velocity at station 0, as is explained in the text in the continuation of line 70:
   The nomenclature is based on one commonly used in turbomachinery, where c indicates absolute velocities
3. The temperature gradient at the wall multiplied by the thermal conductivity of the gas in the boundary layer is an approximation of the convective heat flux. This is a common approach in high supersonic and hypersonic flight at constant Mach number and altitude. The flow and the wall temperature are in equilibrium and so the temperature gradient close to the wall is assumed to be the same as from the fluid to the wall.
4. Both deltac and k depend on the wall temperature, thus, an iteartive approach is required. The equations can be found in references 9 and 10 as is mentioned in lines 61 and 66 respectively.
5. The parameters theta and nb are explained in lines 74 and 75 respectively. The table shows the values used for the cord angle theta and the number of blades used in the parameter sweep to find the efficiency maximum for a 150kW propeller. The other numbers in the table are the cord length of the blade which was necessary to produce the required amount of thrust for the given combination of theta and nb as is mentioned in the table caption. Section 3.1.1 shows how the different parameters affect the power efficiency. The number of blades multiplies the force per blade and, thus, affects the cord length of the final propeller blade if the thrust is constrained.

Concerning the use of the English language:

1. I disagree, the sentence says that if the feasibility of the system can be demonstrated that further and more detailed investigation is warranted. I can rewrite the sentence but it is not wrong.
2. It says 'So too for' in line 5 not 'So too far'. I have added commas, otherwise the sentence is correct.
3. I don't see what is repeated in this sentence.

I have run a spell check on the entire document. There were two mistakes which I have corrected. If you believe that there are more mistakes please point them out as I can't find them.

Kind Regards,

Reviewer 2 Report

Aerodynamic and thermodynamic analysis of a supersonic propeller operating at flight Mach numbers from 2 to 6 between 15 and 35km altitude driven by an electric motor is performed in this paper.

1、  In this paper authors studied three types of blades including flate plate blade, a diamond shaped blade and a generic four-side blade. Are these blades commonly used in propeller? Why do authors investigate these three types of blade?

2、  What do the dash lines in Fig 5b stand for? The deviation between model and experiment is quiet obviously in St number. If the boundary layer transition can be considered in the model the results would be more convinced.

3、  Can propeller operate in Mach number 6? What is the advantage of propeller when it compare to ramjet or scramjet at Mach number 6?

4、  Why do the altitude have greater effect on power efficiency than flight Mach number?

Author Response

Dear Reviewer 2,

Thank you for your comments. Please see my responses below.

1. A flat plate is commonly known to be the most efficient aerodynamic shape in supersonic flow. The downside is that it has no thickness, which can create structural problems. A thick flat plate has a thick blunt leading edge which is very inefficient. This is why supersonic airplanes use diamond cross-sections because they are the second most efficient body in supersonic flow and they can be made thicker to account for structural loads while keeping a sharp leading edge. The generic four-sided blade is a generalisation of the diamond to see if a more complex cross-section can be more efficient than a flat plate. I've added some clarification at line 100, highlighted in red.
2. The dashed lines represent the Stanton number for a turbulent boundary layer. I've amended the legend of figure 5b and added a sentence for clarification in lines 124-126.
   I hope this alleviates the concerns between the model and the experimental Stanton number. While the transition region is not modelled, there is quite good agreement for a laminar and a turbulent boundary layer.
3. According to this analysis, the blade temperature will prevent flight at Mach 6 assuming temperatures above 1500K can't be sustained, see Figure 12b. As this is a feasibility study, a proper comparison to ramjets and scramjets is not within the scope of this study. However, an electrically driven propeller offers the advantage of high torque over the entire operational range of the            electric motor plus the possibility of using the propeller as an active break and, therefore, improved manoeuvrability. No combustion driven aircraft has access to an active in-flight break.
4. I've added the following explanation at line 242242:
   An increase in altitude causes a drop in density and, thus, a thickening of the boundary layer. The thicker boundary layer causes higher viscous drag. This in turn significantly affects the power efficiency at lower velocity ratios where the total forces are lower. The efficiency maximum, thus, moves to a higher velocity ratio, where the angle of attack is higher and the leading edge shock is stronger. The combination of a higher angle of attack with higher viscous drag causes the total reduction in efficiency.
   An increase in Mach number, on the other hand, creates a stronger shock at the leading edge but it increases the Reynolds number at the same time, which reduces the boundary layer thickness and, therefore, the viscous drag. Since the two effects are competing, the reduction in power efficiency is smaller than for the change in altitude.

Kind Regards,

Reviewer 3 Report

This is an interesting concept for supersonic propulsion potential of taking the place of conventional chemical propulsion. There are some comments.

(1) The details of Fig.2 are not clear. The reviewer doesn’t know how the vehicle works. For example, what are the conditions at station 0, 1, 2, 5, 9? What is the Mach number at station 2? Supersonic or subsonic?

(2) The reviewer doesn’t know which part generate thrust? The blade or the nozzle? The author should give a detailed sketch of the flow filed or streamlines in Fig.3 to show the mechanism of how to generate thrust.

(3) The shock-expansion theory used in the analysis should be explained briefly or give the references.

(4) The Eq.(9) to predict the lift-to-drag ratio is usually used for two-dimensional wave riders. However, the vehicle in this paper is axisymmetric.

Author Response

Dear Reviewer,

Thank you for your comments. Please see my responses below

1. The flow conditions are exclusively supersonic except for the boundary layer. Conditions at station 2 are shown in section 3
2. This propulsion system has no nozzle as there is no internal flow. The propeller produces the thrust. The velocity triangles show the local flow direction upstream and downstream of the propeller blade. On the propeller blade itself the flow is parallel to the blade.
3. I have added a reference for the shock-expansion theory.
4. This is not true, Equation 9 is an empirically derived L/D limit for a variety of geometries, wave riders can exceed this limit as mentioned in the text. The equation for wave riders is L/D = 6 (M + 2) / M as can be seen in the cited reference.

Kind Regards

Round 2

Reviewer 1 Report

The manuscript can be accepted as the author has addressed all the queries well.

Reviewer 3 Report

The comments are answered. 

In addition, I did not express my question clearly in the last review. Usually, we do not think that the propellers are good for supersonic flight because it will induce big shock wave drag. The authors does not discuss this point of view in the paper and should consider this problem in the future work.