DESIGN ANALYSIS for a SMALL SCALE ENGINE by Tim van Wageningen

DESIGN ANALYSISfor a

SMALL SCALE ENGINE

by Tim van Wageningen

Contents

- Motivation- Concepts- Performance Analysis- Conclusions- Questions

±40 min

2 MOTIVATIONS - CONCEPTS – PERFORMANCE I / II / III - CONCLUSIONS

Nature

Technology

scale →

small large

Atalantaproject


Micro Air VehicleFlapping Wing Mechanism

- Designed by Casper Bolsman- 0.6 gram- Performance estimate:

- 0.5 W power output

- Needed power density of

system: 125 W/kg

- 6 minutes of flight time with

5% efficiency

MAV


MAV in Action


Hydrogen Peroxide

- Master thesis of Arjan Meskers at the PME department, TU Delft

- Chemical energy: high energy density

- Monopropellant

- Clean products: oxygen and water vapor

- Example catalysts: -Manganese oxide

-Silver-Platinum


Catalytic Reaction in Action


Thesis Assignment

Find an engine concept that:- is suitable for the MAV

- 125 W/kg- 5% efficiency

- uses hydrogen peroxide as fuel


Possibilities

9 MOTIVATIONS - CONCEPTS - PERFORMANCE I / II / III - CONCLUSIONS

3 different approaches

Turbine Piston Cylinder

+

+

+ +

+


Concept I: Tesla Turbine Engine

+ Easy implementation

+ Theory of Tesla Turbine predicts good efficiency at small scale

- Conversion from rotation to linear motion

+

+



+

+


Concept II: Otto Engine

+ Proven concept on regular scale

- Projects in literature show bad performance because of fluid leakage problem

- Implementation difficult

+


Concept II: Otto engine

+


Concept III: Hot Air Engine

+ Easy implementation

+ Promising scaling aspects because heat transfer is more effective

- Poor performance on regular scale

+

+



+

+


Performance

- What influences the performance of these concepts?

- Concept I- Concept II- Concept III

- Are the concepts suited for the MAV?- Power density- Efficiency




Concept I: Tesla Turbine Engine:model

Assumptions:Laminar flow

No entrance effects

Incompressible fluid


Power Efficiency

Pressure difference

Length of belts(radius of discs)

Height of gap(spacing between discs)


[2] V.G. Krishnan et al. A micro Tesla turbine for power generation from low pressure heads and evaporation driven flows. Transducers, 11:1851 – 1854, June 2011.

Measurements with small scale Tesla turbines

Pressure difference:~20 kPa

Measured Performance45 mW

18% efficiency

Estimated power density:2 W/kg


- Power density is too low: pressure difference must be increased considerably

- Simple model + measurements show that TTE is not suitable for the current size MAV

Concept I, Tesla Turbine Engine:conclusions


Concept II: Otto Engine


Concept II, Otto Engine: combining 3 models

Catalytic Reaction

Heat Loss

Exhaust Flow + +


Catalytic Reaction: model


Drop on a catalytic surface

Similar conditions as during experiments

Energy Balance:

Catalytic Reaction: model

[1] A.J.H. Meskers. High energy density micro-actuation based on gas generation by means of catalyst of liquid chemical energy. Masters thesis, TU Delft, 2010.


Catalytic Reaction: high fuel concentrations


Exhaust Flow: model

Compressible flow through a

round nozzle

Based on momentum

equation


Heat transfer

Heat is transferred via

-conduction-convection-radiation


Concept II, Otto Engine: combining models

+ + =

- Dealing with model uncertainties:


Otto Engine: observations

-Reaction times are fast enough-Trade off for fuel used per cycle

-Condensation in cylinder


Concept II, Otto Engine: Results

- Model shows performance above the current requirements of the MAV (125 W/kg @ 5% efficiency)


Concept II, Otto Engine: considerations

- Model neglects:- fluid leakage through cylinder/piston gap

- fluid friction at exhaust- fuel delivery system

- Condensation in cylinder problem needs to be addressed




Concept III, Hot Air Engine:models

+ +

Catalytic Reaction

Heat Loss

Heat Reservoir

s


Concept III, Hot Air Engine:Catalytic Reaction

Constant temperatureMass balance


Concept III, Hot Air Engine:Heat Reservoirs

Schematic

Under reversibleconditions

Estimate for heat transfer rates

- Using definition Fouriers law

-Optimistic and pessimistic value


Model Results

Resulting performance of model

+ + =


Considerations for Small Scale Hot Air Engine

- Model neglects losses of - fluid flow between piston cylinder gap

- heat leakage of Decomposition Unit to the working fluid


Conclusions for Small Scale Hot Air Engine

- Heat transfer is not yet fast enough on this scale, which results in low performance


- Concept III is not suited for the MAV

Overall Conclusions

- Of the considered possibilities, the small scale Otto engine is the best option for the MAV:

Power density at 5% efficiency:Concept 1: << 2 W/kg

Concept 2: 245 – 440 W/kgConcept 3: 0.5 – 8 W/kg



Overall Conclusions

Actual implementation

of concept II requires more detailed

analysis:

- Solving the fluid leakage problem

- Fuel pump- Exhaust port

- Condensation

43 MOTIVATIONS - CONCEPTS - PERFORMANCE I / II / III - CONCLUSIONS - END

Thank You!

44 DETAILED SLIDES

Detailed slides

16 PERFORMANCE

Scaling?

Engine 1S = 1L = 10A = 10V = 10

Engine 2S = 0.5L = 5A = 2.5V = 1.25

Scaling factor

LengthArea

Volume

6 PREMILAIRY RESEARCH

Approach of others?

7 PREMILAIRY REASEARCH

Possibilities

40 PERFORMANCE

Power Efficiency

Pressure difference

Length of belts(radius of disks)

Height of gap(spacing between disks)

7 CONCEPTS

Energy flow in concepts

Carnot cycle =

8 PERFORMANCE

Carnot Cycle

zero power output!

9 PERFORMANCE

Curzon Ahlborn Cycle

10 PERFORMANCE

Curzon Ahlborn Cycle

11 PERFORMANCE

ND Curzon Ahlborn Cycle

17 PERFORMANCE

Basic thermodynamic engine model

- Two constant temperature reservoirs:

- Energy flows modeled with Fouriers law of heat conduction:

-Carnot cycle between the working fluid temperatures:

ND Curzon Ahlborn Cycle

18 PERFORMANCE

19 PERFORMANCE

Scaling of performance

20 PERFORMANCE

Intermediate Conclusions

- Efficiency of engine is independent of scale, if the cycle time is adjusted correctly

- Optimal power output can be found by finding the optimal cycle time

- Assuming an optimal engine configuration:

12 PERFORMANCE

Energy Balance Model


13 PERFORMANCE

14 PERFORMANCE


13 PERFORMANCE

Scaling of optimal cycle time concept 3

opti

pessi

16 DETAILS

Heat transfer

- Heat is transferred via -conduction-convection-radiation

- FEM model in COMSOL

Heat transfer: FEM model results

16 DETAILS

16 DETAILS

Heat transfer: facts for MAV engine

- Low Biot number situations: not much use for insulation.

- Difficult to maintain a temperature difference within the system

- Loss term scaling exponent = 1.5

Intermediate Conclusions

15 PERFORMANCE

- The performance of depends on a potential and the utilization

- Utilization is independent of scale

- How does this apply to the concepts?

16 DETAILS

Catalytic Reaction: fundamentals- Decomposition rate proportional to the

effective contact area between fuel and catalyst

- Large Damköhler number: rate temperature independent

- First order reaction:

32 DETAILS

Exhaust Flow: model

Flow through a

nozzle

Based on momentum

equation

Neglects friction

Exhaust Flow: characteristics

33 DETAILS

Model Results: scaling

24 PERFORMANCE

Assuming unrestricted cycle time!

41 PERFORMANCE

What about scaling?

Catalytic Reaction: Fluid Flow:

Power Density at reference scale (S=1):

Power: Power Density:

Power Density when size is 10 times smaller (S=0.1):

Documents

DESIGN ANALYSIS for a SMALL SCALE ENGINE by Tim van Wageningen