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SPCDC: A USER-FRIENDLY COMPUTATIONAL TOOL FOR THE DESIGN AND REFINEMENT

OF PRACTICAL PULSE COMBUSTION SYSTEMS

SPCDC: UN CODE DE CALCTJL FACILE D'UTILISATION POUR LA CONCEPTION ET L'AMELIORATION

DE CHAMBRES DE COMBUSTION PULSEE FONCTIONNANT EN MILIEUX PRATIQUES.

Pamela K. Barr, Jay 0. Keller Sandia National Laboratories, USA

James A. Kezede Gas Research Institute, USA

ABSTRACT

This paper reports on the development and use of a user-friendly, PC- executable computer code that can assist engineers in designing pulse combustors for specific applications and in refining existing units. This code represents the culmination of over 10 years of research and development in the field of pulse combustion. The Sandia Pulse Combustor Design Code, or SPCDC, couples both the fuel-air injection and the energy release to the time-varying pressure wave. Because the injection and combustion processes both drive and are driven by the wave dynamics, this model couples the major processes that occur in a pulse combustor. SPCDC can supplement the tirne-proven method of actually building and testing a prototype unit, and significantly reduce the number of units that must be tested. It will help produce a superior pulse combustion system tailored to a specific application and should help widen the range of successful applications.

RESUME

Cett publication represente le dkveloppement d'un code de calcul permettant d'assister l'inghieur d'etudes dans la conception de nouvelles chambres de combustion pulske fonctionnants en milieux pratiques ainsi que dans l'amelioration de celles existants deja. Ce code represente l'accumulation de plus de dix ans d'expkrience dans le domaine de la combustion pulske. Le Sandia Pulse Combustor Design Code, ou SPCDC, couple l'injection carburant/comburant et le dkgagement de chaleur avec l'onde de pression instationnaire. Parce que l'injection et le processus de combustion gouvement et sont gouvern6s tour A tour par cette onde, ce mod2le couple les phenom2nes majeurs inherents B la combustion pulsee. Le SPCDC fournie une mkthode eprouv6e pour la construction et le test de prototype qui devrait rkduire le nombre d'unitks B tester. Ce mod& devrait aider B la production de meilleurs systGmes, chacun d6fini et congu pour une application prkcise, donc devrait ouvrir des champs d'applications pratiques pour la combustion pulsee.

DISCLAIMER

This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, make any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

DISCLAIMER

Portions of this document may be illegible in electronic image products. Images are produced from the best available original document.

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INTRODUCTION

. Pulse combustors are devices that are designed purposely to release heat in an oscillatory mode. Successful applications of these devices have taken advantage of these benefits, primarily for fluid heating. Pulsating combustion achieved credibility as a practical technology in the 1980’s with the successful marketing of the Lennox PulseTM warm-air furnace and Hydrotherm’s HydroPulseTM hydronic heating unit for residential space heating. Subsequently, Fulton Boiler Works in the US. and Pulsonex in Sweden have applied the technology to commercial-scale boilers.

Another application of pulse combustion is spray drying, where the hot, oscillating exhaust flow atomizes a liquid stream into fine droplets, without using nozzles or rotary atomizers, accelerating drying rates. BFpex Corporation claims that their UnisonTM Drying System produces a superior product to that produced by traditional spray dryers.

There are many advantages of pulse combustors. The heat transfer is enhanced in pulse combustors by the large oscillations in the flow field. Dec and Keller (1989) have measured heat-transfer rates up to two and a half times greater than those obtained for steady turbulent flows at the same mean Reynolds numbers. The enhanced heat transfer means that a smaller furnace can be used to provide the same energy output. Pulse combustors develop positive pressures and therefore do not require expensive auxiliary equipment such as external blowers or flues. The pressure oscillations enable them to draw in their own supply of fuel and air, and the resulting thrust is used to vent the exhaust products through a small diameter plastic pipe, which allows more flexibility in siting the unit. The maintenance expenses are low because there are few movable parts. Another advantage of gas-fired pulse combustors is the low NO, level in the exhaust gas obtained without additional pollution control equipment. NO, emissions as low as 3 ppm have been measured from practical size units (Keller, et al., 1994). The main disadvantages of pulse combustors are noise and vibrational issues. However, the combustor‘s physical structure can be designed to reduce these problems.

Pulse combustors are not burners, but complete combustion and heat transfer systems, and, thus, require a more complex design process. A major deterrent to the utilization of pulse combustion devices in the past was the inability to predict a priori the performance and stability of a particular design. The source of this difficulty was the complex coupling of the two processes that control pulse combustor performance: the resonant acoustic pressure field and the large, transient energy release. Seemingly minor changes in design or operating conditions could lead to unexpected changes in operation. Fundamental research supported and directed by US DOE and GRI defined how the underlying chemistry and fluid mechanics control these processes, making possible new tools for designing and optimizing pulse combustion devices. Design handbooks, such as can be found in Vishwanath (1985), provide a systematic approach to the design of a Helmholtz-type pulse combustor for a particular application. However, handbooks do not allow one to investigate the response of a particular design to changes in geometry, the effects of scaling a particular design, or the implications of operating a pulse combustor with an unusual fuel composition. A new design tool was needed to help extend pulsating combustion to new applications such as thermal fluid heaters, unit heaters, and absorption cooling systems.

The Sandia Pulse Combustor Design Code, or SPCDC, simuliites an entire pulse combustion system, from the intake line to the exhaust decoupler, as shown in Fig. 1. It can investigate the impact of the intake line lengths required for different installations. It can explore the feasibility of input power turn-down options. It can be

combustion chamber

exhaust decoupler

Figure 1. Schematic of a Helmholtz-type pulse combustor, showing the components that are included in SPCDC.

used to compare sound power levels emitted from pulse combustors running at different operating conditions.

Currently, SPCDC is being used by both Denver Collins of State Industries and Lance Evens of Lennox Industries to design pulse combustors for fluid heating applications. With this model they have been able to predict many of the experimental effects observed from their pulse combustors, including the influence of system geometry, heat transfer, reactant supply pressure, and other combustor operating conditions. With SPCDC they can optimize a design for specific applications, or scale a design to a desired input-power rate.

PULSE COMBUSTOR OPERATION

All pulse combustors use a periodic combustion process to drive a resonant pressure wave. The design under consideration here is known as a Helmholtz-type pulse combustor, shown in Fig. 1. It consists of a closed cylinder, which acts as the combustion chamber, which is attached to a long tail pipe through a transition section. Fresh charge is introduced into the combustion chamber through either a flapper valve or an aerodynamic valve. Both types of valves allow the gas to flow in one direction; the flow in the other direction is blocked mechanically in the flapper valve, and it is strongly resisted by the geometry of the aerodynamic valve. The exhaust decoupler shown in the figure is not needed to support the oscillations, rather it is used in commercially available systems to terminate the acoustic wave, thereby suppressing sound emission.

In 1878, Rayleigh identified a fundamental requirement for stable combustion- driven pressure oscillations (Rayleigh, 1878). He pointed out that for the combustion process to drive pressure waves, the energy release must be in phase with the pressure cycle, which is a function of the system geometry and operating conditions. This phasing for a pulse combustor is shown in Fig. 2. Rayleighs criterion states that the maximum pulsation strength will occur when the pressure wave and the energy release profile are entirely in phase with each other. In other words, timing is everything.

The time required for the pressure wave to travel through the system can be quantified by the characteristic resonance time, which-for a given geometry-is primarily a function of temperature. The timing of the heat release is a much more complicated function of flow, mixing, and chemistry. The cold reactants, which are injected during the low-pressure portion of the cycle, will not burn until several things have occurred. First, the fuel and air must mix locally to a concentration that can burn. This mixing process is controlled in part by the fluid dynamics of the injection

c

I ' I ' I ' 1 ' ~4 16 - Pressure -

Energy Release Rate -- - Reactant Flow Rate - - - - - h

! ! , . . , 0.0 0.2 0.4 0.6 0.8 1.0

Cycle Time (t / t cycle)

1 10

m 5

-1 0

Figure 2. Cycle-resolved profiles showing the typical relative timing of the pressure wave, reactant flow, and energy release in the combustion chamber.

process. Fluid dynamics also controls the rate at which the cold combustible gas mixes with the hot products, heating it to a temperature at which it can react. Finally, a finite amount of time is then required for the hot reactants to initiate and complete the chemical kinetics of combustion. The critical part of pulse combustor design involves ensuring that the energy release will occur during the high-pressure portion of the cycle. Although injection of the fresh charge is tied to the pressure wave, the processes that control when the combustion energy is released are different from the processes that control when the pressure peaks.

Because the pulsation strength and stability are sensitive to the relative timing of the pressure wave and the energy release profile, an enhancement of the oscillation strength and stability can be obtained by tuning the energy release profile to the acoustic wave. This can be accomplished by changing any one of the characteristic times (flow, mixing, or chemical reaction) that controls either the periodic combustion or the acoustics (Keller, et al. 1989a and 1989b, Barr, et al. 1990).

MPUTATIONAL Ti L

The Sandia Pulse Combustor Design Code has grown out of knowledge of these controlling processes gained from over ten years of research and development funded by DOE, GRI, and others. The purpose of SPCDC is to simulate the performance of different pulse combustor designs under a variety of operating conditions to help identify desirable combustor configurations for specific residential, commercial and industrial fluid heating applications. This approach should reduce the number of experimental prototype units that must be built and tested in the laboratory, yet will allow evaluation of many more designs than could be without the aid of a computer model. SPCDC is intended to be used in conjunction with other design tools by engineers working to improve their fluid heating systems (or absorption cooling systems) via the application of pulse combustors.

SPCDC is based on a noidinear numerical model that was developed as a part of a program funded by the U. S. Department of Energy, Office of Industrial Technologies. It was originally developed as a research tool, but under GRI funding, extensions have been added to the model to simulate components used in practical pulse combustor

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systems and the code has been made user-friendly through the use of a graphical user interface (GUI).

Model Basis

In the model the pressure distribution throughout the unit is determined from the wave dynamics of the system. They are simulated by solving the one-dimensional nonlinear transient equations of continuity, momentum and energy (Barr and Dwyer, 1991). These equations are integrated using MacCormack’s explicit finite difference method (MacCormack, 1969), which has been shown to accurately simulate fluid flow problems involving wave dynamics.

The processes that control the periodic energy release both drive, and are driven by, the time-varying pressure wave (see Barr, et al., 1990, for details of the model). This interdependency, which forms the basis of SPCDC, is depicted in Fig. 3. A stable solution is characterized by pulsations with no cycle-to-cycle variation. This indicates that the pressure wave from one cycle can produce an energy release profile, through the logic shown in Fig. 3, that can maintain the pressure wave without changing its shape, including amplitude and time scale.

The equations include terms for frictional losses, heat transfer, and energy release from combustion. The wall friction is represented in the momentum equation in terms of the standard friction factor for quasi-steady flow. The heat transfer rate to the load is computed using a relation for the heat transfer coefficient derived for oscillating flows, such as those in pulse combustors (Arpaci, et al., 1993). Although the periodic combustion process is included as a single term in the energy equation, it controls performance of the pulse combustor, making its characterization critical.

The energy release profile is simulated using a three-step process: 1) reactant injection through the valve, 2) mixing of these cold reactants with hot combustion products, and 3) chemical reaction for release of the energy. It is assumed that fuel and air enter premixed. In the first step, the time-varying pressure difference across the valve is used to determine the profile of reactant injection. The valve coefficients have been set by the user to simulate a particular commercial or experimental valve.

f Combustion chamber pressure determines the injection profile

I I Mixing rate + chemical kinetics

determine the energy release J L J

Injection determines the rate o mixing of the cold reactants

with the hot products

Figure 3. Depiction of the interdependenaes of the wave dynamics, injection, mixing, and combustion processes, which are incorporated into SPCDC.

The second step models the injection of the reactants into the hot products inside the combustion chamber. Ignition of these cold reactants will not occur until they mix with enough of the hot products to increase their temperature and/or the radical concentration to ignition values. The fluid dynamic mixing is a complex three- dimensional process, which is configuration-dependent. SPCDC simplifies this by treating the injection as a single unconfined jet. This simplification captures the experimentally observed behavior of most practical pulse combustors.

The final step in producing the energy release profile simulates the time needed for the chemical kinetics to occur. This ignition delay time depends on the reactant mixture, including the fuel-air ratio, the fuel mixture, and the amount of exhaust gas recirculation. SPCDC is connected to a chemical kinetics software package to determine the necessary parameters that characterize a user-specified reactant mixture. This package is based on a well-stirred reactor model (Westbrook, et al., 1988).

User-Interface

The user-interface to SPCDC is a Windows program running on a Microsoft Windows 3.1 platform. The pull-down menus in the application window bring up dialog boxes that allow the user to select the desired input file, to modify values for each of the parameters (as well as to change dimensional units), and to set up and run either a single study or a parametric design study. Examples of these are shown in Figs. 4-6. The output from SPCDC can be analyzed with most commercially available spread sheet or graphical software, such as Microsoft Excel, Lotus 1-2-3, or Borland Quattro Pro.

Length -Diameter

Intake Line p i l l f t j g Valve Box 10.351 a D Inlet Line

Combustion Chamber mn Transition Section 17-q Number of

Tail Pipes

Tail Pipe pii7l-J Decaupler 112.51 Number of

Vent Pipes

El Vent Pipe

Figure 4. SPCDC application window containing the dialog box used to set the physical dimensions of the pulse combustor system. These dimensions can be specified using a combination of units (eg., inches, feet, m e w , and mm), which are selected through the drop-down list boxes adjacent to each parameter.

Estimated Frequency I----lM] 100

Input Power v I l B t u / h r

Figure 5. The dialog box used to specify characteristics of the fuel-air mixture. The fuel composition can be selected from a predefined set (methane, propane, or a mix of 90% methane and 10% ethane, which we have named generic natural gas). Or SPCDC can generate the properties for a user-specified fuel mixture to account for variabilities in natural gas composition.

....................................................... Distance $n Temperature F

...................... ...........

Veloaty [fvs

Density I b din-3

Pressure

Pressure rms in ~ 2 0 [a]

psig

flow Rate lhmfir

Energy -1

Power Btu/hr

Frequency H~

Figure 6. The dialog box used to set the dimensional units for the output from SPCDC. The user sets units for each type of dimension individually, rather than selecting one system of units (such as SI or English).

EXAMPLE OF A DESIGN STUDY

As an example of the usage of SPCDC, consider the design problem of selecting the optimal number of tail pipes that should be used in a new system. We want to obtain the maximum thermal efficiency, constrained by the requirement that the design produces strong, stable pulsations. For simplicity, we will fix all other design and operating parameters. Values of the main parameters for this example are listed in Tables I and IL In SPCDC values for these parameters are selected through the pull- down menus.

* SPCDC can quantify the impact of adding tail pipes to the pulse combustor system. Thermal efficiency should continue to increase with increases to the available surface area for heat transfer (Le., the net surface area of all of the tail pipes), but the frictional losses increase for smaller diameter tail pipes. The requirement of strong stable pulsations indicates that there could be a trade-off between thermal efficiency

x

... ... . . . . ....

.... . _ . . . . . . . . . ..

and operational stability. Trade-offs between thermal efficiency and other parameters (material cost, system size, noise, etc.) could also be analyzed.

The results from SPCDC, presented in Fig. 7, show that the operating frequency decreases as the number of tail pipes increases. To keep the net cross-sectional area of the tail pipes constant, the diameter of each decreases as we increase the total number. This also results in both an increased surface area for heat transfer and greater frictional iosses caused by smaller diameters of each pipe. Figure 7 shows that as tail pipes are added, the frequency drops because the gas temperature within the combustor loses more energy through the additional surface area. The drop in frequency is accompanied by an increase in the heat transfer efficiency. Other results show that the pressure oscillations remain strong and stable over this range, and that the mean combustion chamber pressure increases as tail pipes are added to overcome the increased frictional losses so that the burned gas can be exhausted. The heat transfer efficiency levels off once there is sufficient surface area to allow the gas in the tail pipe to reach the external temperature, and so additional tail pipes do not enhance the heat transfer. When considering manufacturing costs, these results show that the optimal number of tail pipes probably is less than ten, because the thermal efficiency does not increase dramatically beyond that number, unlike manufacturing costs.

Table I Combustor Geometry Geometrical Component Length (m) Diameter (m)

combustion chamber

transition section

0.10 0.08

0.10 conical

tail pipe (diam. is for one tail pipe) 0.88 0.03

exhaust decoupler 0.125 0.125

vent pipe 0.30 0.03

Table 11. Other Parameters

combustible mixture natural gas and air, $= 0.75

input power 15 kW

reactant temperature 300 K

external temperature 300 K

heat transfer coefficients: 60 W/(m2K) combustion chamber empirical relation tail pipe 60 W/(m2K) exhaust decoupler 60 W/(m2 K) vent pipe

friction coefficient

back pressure

supply pressure

injection system: inlet diameter mixing coefficient

valve characteristics: cross-sectional area length friction coefficient

0.04

100 kPa

104 kPa

0.0254 rn 0.007

510 mm2 115 mm 3.4

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Number of Tail Pipes

Figure 7. Results from the design study of number of tail pipes. As the number of tail pipes increases, the frequency drops due to the drop in the internal temperature, and the efficiency levels off when the temperature of the exhaust gas approaches the external temperature.

AVAILABILITY OF SPCDC

For additional information regarding technical issues or code distribution contact Pamela Barr at the Combustion Research Facility, Sandia National Laboratories, Livermore, California, 94551-0969, USA.

SUMMARY

SPCDC is being developed with guidance from engineers representing companies that manufacture pulse combustion systems. It's predictions have been compared with data from commercial and prototype hardware of several types and sizes. It draws upon many aspects of pulse combustion research and development, including fundamental experimental research, multidimensional transient computations, and extensive development for specific applications. SPCDC has evolved from a research tool developed to help understand the coupling between the acoustics, the injection, and the energy release for a bare-bones combustor configuration into a user-friendly engineering tool. Many algorithms have been added to SPCDC to make it useful to heating equipment manufacturers.

SPCDC simulates an entire pulse combustion system, from the intake line to the exhaust decoupler. Although SPCDC will never replace the time-proven method of actually building and testing a unit, it can significantly reduce the number of units that must be tested. And it can help produce a superior pulse combustion system tailored to a specific application. When used by heating system designers, it will help bring the inherent advantages of pulse combustion systems-high efficiency, lower emissions without external controls, improved venting, and lower manufacturing costs-to a wider range of residential, commercial and industrial fluid heating systems.

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ACKNOWLEDGEMENTS

We would like to thank Denver Collins of State Industries and Lance Evens of Lennox for providing us with an industrial perspective during development of SPCDC. Charlie Westbrook of Lawrence Livermore National Laboratory has been instrumental in the development of the chemical kinetics submodel. John Dec of Sandia has provided continued assistance with the heat transfer correlation. We are grateful to Taz Bramlette of Sandia for his contributions to various aspects of this project.

This work was performed at the Combustion Research Facility, Sandia National Laboratories, and supported by the Gas Research Institute, Chicago, Illinois, USA. SPCDC is based on a research code developed at Sandia under funding by the U. S. Department of Energy, Office of Industrial Technologies.

REFERENCES

Arpaci, V. S., Dec, J. E., and Keller, J. 0.: “Heat Transfer in Pulse Combustors,” Combustion Science and Technology, Vol. 94, p. 133 (1993).

Barr, P. K., Keller, J. O., Bramlette, T. T., Westbrook, C. K. and Dec, J. E.: ”Pulse Combustion: Demonstration of the Importance of Characteristic Times,” Combustion and Flume, Vol.82, p. 252 (1990).

Barr, P. K., and Dwyer, H. A.: “Pulse Combustor Dynamics: A Numerical Study,“ Chapter 22 in Numerical Amxoaches to Combustion Modeling (E. S. Oran and J. P. Boris, eds.), American Institute of Aeronautics and Astronautics (AIAA) Progress Series, (1989).

Dec, J. E., and Keller, J. 0.: ”Pulse Combustor Tail-Pipe Heat-Transfer Dependence on Frequency, Amplitude, and Mean Flow Rate,” Combustion and Flame, Vol. 77, p. 359 (1989).

Keller, J. O., Dec, J- E., Westbrook, C. K. and Bramlette, T. T.: “Pulse Combustion: The Importance of Characteristic Times,” Combustion and Flame, Vol. 75, p. 33 (1989a).

Keller, J. O., Bramlette, T. T., Dec, J. E. and Westbrook, C. K.: “Pulse Combustion: The Quantification of Characteristic Times,” Combust. Flame, Vol. 79, p. 151 (1989b).

Keller, J. O., Bramlette, T. T., Barr, P. K. and Alvarez, J. R.: ”NOx and CO Emissions from a Pulse Combustor Operating in a Lean Premixed Mode,” Combustion and Flame, Vol. 96, p. 460 (1994).

MacCormack, R. N.: “The Effect of Viscosity on Hypervelocity Impact Cratering,” AIAA Paper 69-354, Cincinnati, OH (1969).

Rayleigh, J. W. S.: Nature, Vol. 18, p. 319 (1878); also The Theory of Sound, Vol. 2, p. 226, Dover Publications, New York, 1945.

Vishwanath, P. S.: “Advancement of Developmental Technology for Pulse Combustion Applications” Gas Research Institute Report No. GRI-85/0280, Chicago, IL (1985).

Westbrook, C. K., Pitz, W. J., Thorton, M. M. and Malte, P. C.: “A Kinetic Modeling Study of N-Pentane Oxidation in a Well-Stirred Reactor,” Combustion and Flame, Vol. 72, p. 45 (1988).