WO2019147942A1

WO2019147942A1 - Method for supressing surge instabilities in turbomachine compressors

Info

Publication number: WO2019147942A1
Application number: PCT/US2019/015158
Authority: WO
Inventors: Richard DEHNER; Ahmet Selamet; Keith Miazgowicz
Original assignee: Ford Global Technologies LLC; Ohio State Innovation Foundation
Current assignee: Ford Global Technologies LLC; Ohio State Innovation Foundation
Priority date: 2018-01-25
Filing date: 2019-01-25
Publication date: 2019-08-01
Anticipated expiration: 2020-07-25

Abstract

A compression system (10) for an engine includes a compressor (14) and an inlet duct (12) operatively coupled to the compressor (14). The inlet duct (12) has an inlet duct cross-sectional area, Ad. A restriction (24) is positioned within the inlet duct (12) and the restriction (24) has a cross-sectional flow area, Ar, which is equal to or less than Ad and is selectively variable. A controller (26) is adapted to selectively vary Ar as a function of a mass flow rate of gas through the compressor (14). The controller (26) is programmed to operate the restriction (24) to reduce Ar based on the mass flow rate of gas through the compressor (14) to prevent a surge condition in the compressor (14) as the mass flow rate of gas through the compressor (14) decreases below a surge stability limit defined at Ar=Ad at a current compressor rotational speed.

Description

METHOD FOR SUPPRESSING SURGE

INSTABILITIES IN TURBOMACHINE COMPRESSORS

RELATED APPLICATION

[0001] The present application claims the filing benefit of U.S. Provisional Patent Application No. 62/621 ,812 filed January 25, 2018, the disclosure of which is hereby incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

[0002] The invention relates generally to turbomachine compressors and, more specifically, to suppressing surge instabilities in turbomachine compressors.

BACKGROUND OF THE INVENTION

[0003] Turbochargers are frequently used on internal combustion engines to improve the engine’s power output and fuel efficiency. Turbochargers contain a turbine coupled to a compressor via a shaft. The engine’s exhaust gas spins the turbine which spins the compressor via the shaft. The spinning compressor compresses the intake air entering the engine’s combustion chamber. By compressing the intake air, the compressor forces more air into the combustion chamber to enhance the combustion process, causing the engine to make more power compared to the same engine without a turbocharger. The increase in engine output is related to how well the turbocharger performs over a wide range of operating conditions.

[0004] The performance of axial and centrifugal compressors is typically represented in terms of pressure increase through the machine as a function of flow rate at constant rotational speeds N. When the pressure increase is expressed as the ratio of static outlet pressure p₂ relative to total (stagnation) inlet pressure p_{t l}

(total-to-static pressure ratio) PR Pz

c,ts

Pt.i (1 ) compressor performance curves may be utilized to assess system stability. A representative compressor characteristic 2 at constant A/ is shown in terms of PR_c,t_s vs. compressor mass flow rate m_c in Fig. 1 . Alternatively, the compressor rotational speed and mass flow rate may be corrected for fluid total temperature T_{t l} and p_{t l} a„he compressor inle,

Pref respectively, where Tref and p_ref are the reference temperature and pressure, respectively.

[0005] Operation of a compressor on the negatively sloped (where slope, i.e., the dPRr ts

rate of change, = -— ) region (i.e., to the right of line 4 in Fig. 1 ) of its constant dm_c

rotational speed compressor characteristic 2 is unconditionally stable, regardless of the compression system in which it is installed. The stable nature of the compressor is apparent by considering a small perturbation from operation at position 6 in Fig. 1 that decreases the flow rate through the compressor, which leads to an increase in the compressor pressure ratio (outlet pressure) and a reduction in the pressure drop across the downstream restriction (“Stable” curve). As a result, the flow accelerates at the compressor outlet and the flow rate increases until equilibrium is restored at the original operating point (position 6).

dPR_r†S

[0006] As the slope (rate of change),—— , approaches zero, the pressure ratio όΎϊC^

reaches its maximum value (for a given turbocharger speed) and a further reduction in mass flow causes the compressor to operate on the positively sloped region (i.e., to the left of line 4 in Fig. 1 ) of its constant rotational speed compressor characteristic 2, where surge instabilities can occur. Surge is a global instability, resulting in pressure and flow rate fluctuations throughout the compression system. The frequency of mild (also referred to as soft) surge is dictated by the natural frequency of the compression system. Mild surge instabilities typically occur as the local slope of the (total-to-static pressure ratio vs. mass flow rate) constant rotational speed compressor characteristic 2 reaches a positive value (/^'.e., to the left of line 4 in Fig.

1 ), and the amplitude of mass flow rate and pressure fluctuations increases with decreasing flow rate. Further reduction in flow rate results in deep surge, which is characterized by severe oscillations and cross-sectional averaged flow reversal during part of the cycle. It is unacceptable to operate a compressor in deep surge because it is a severe instability, which results in drastically increased levels of noise, loss of pressure rise, and potential mechanical failure. Therefore, the boundary between mild and deep surge operation marks the low-flow operating limit of a compression system.

[0007] Surge instabilities have limited the low-flow operating range of centrifugal compressors since their discovery over 300 years ago. Research from 1955 provided a fundamental description of surge physics by capturing velocity

fluctuations throughout the entire compression system ducting. The research also laid the groundwork for future developments by identifying that fluctuations due to surge instabilities occur near the natural frequency of the compression system, which corresponds to the Helmholtz resonance frequency for the typical duct-plenum (volume) ducting configuration shown in Fig. 2. The compression system 10 in Fig. 2 includes an inlet duct 12, a compressor 14, and an outlet duct 16, a plenum 18, a throttle duct 20, a throttle mechanism 22 positioned inside the throttle duct 20. [0008] One researcher demonstrated, through linearization of the equations of his lumped model, that compression system surge instabilities are possible when the dPRr ts

compressor operates on the positively sloped -— portion of the constant

dm_c rotational speed total-to-static pressure ratio PR_{C ts} vs. mass flow rate m_c compressor characteristics.

[0009] Numerous studies have explored both passive and active methods to modify the flow-field near the compressor inlet to mitigate compression system surge instabilities and extend the usable portion of the low-flow operating range. These flow-field manipulation approaches have consisted of compressor inlet guide vanes, inducer inlet flow field modifications, flow recirculation both with and without jets to induce aerodynamic pre-whirl, and casing treatments (included ported shroud compressor covers). Other approaches have utilized lumped compression system models to implement and evaluate the performance of active control to suppress surge instabilities. These studies have explored the ability to suppress the growth or cancel fluctuations at the surge frequency utilizing a fast acting valve in the compressor outlet duct or within the plenum to bleed compressed air, along with a moveable plenum wall or a speaker.

[0010] None of the prior studies, however, have utilized a restriction installed in the inlet duct 12 to stabilize the compression system 10.

SUMMARY OF THE INVENTION

[0011] In one embodiment, a compression system for an engine comprises a compressor with a compressor inlet and a compressor outlet. An inlet duct is operatively connected to the compressor inlet and the inlet duct has an inlet duct cross-sectional area, Ad. An outlet duct is operatively connected to the compressor outlet. A restriction is positioned within the inlet duct and the restriction has a cross- sectional flow area, A_r, wherein A_r is equal to or less than Ad. In one aspect, an area ratio, ARO, is defined by Ar divided by Ad, Ar being selectively variable such that ARO is variable in the range 0 < ARO < 1.

[0012] In one embodiment, A_r is selectively variable and the compression system further includes a controller adapted to selectively vary A_r as a function of a mass flow rate of gas passing through the compressor (14). In one aspect, the controller selectively varies A_r such that ARO varies in the range 0 < ARO < 1 . In another aspect, the controller is programmed to operate the restriction to reduce Ar based on the mass flow rate of gas passing through the compressor to prevent a surge condition in the compressor as the mass flow rate of gas through the compressor decreases below a surge stability limit defined at Ar = Ad at a current compressor rotational speed, where the surge condition is characterized by a fluctuation in the mass flow rate of gas through the compressor.

[0013] In another embodiment, a compression system for an engine comprises a compressor with a compressor inlet. An inlet duct is operatively connected to the compressor inlet and the inlet duct has an inlet duct cross-sectional area, Ad;. A restriction is positioned within the inlet duct and has a cross-sectional flow area, Ar, which is equal to or less than Ad and is selectively variable. The compression system further includes a controller adapted to selectively vary Ar as a function of a mass flow rate of gas passing through the compressor. The controller is

programmed to operate the restriction to reduce A_r based on the mass flow rate of gas passing through the compressor to prevent a surge condition in the compressor as the mass flow rate of gas through the compressor decreases below a surge stability limit defined at Ar = Ad at a current compressor rotational speed. The surge condition is characterized by a fluctuation in the mass flow rate of gas through the compressor.

[0014] in one aspect, the compressor defines a compressor characteristic of total- to-static pressure ratio, PR_c,t_{s >} versus mass flow rate, m_c, for a constant rotational speed of the compressor, wherein the compressor will encounter the surge condition when a rate of change, , of the compressor characteristic is positive. The

compressor characteristic for any one rotational speed of the compressor has positive rate of change to a threshold mass flow rate of gas through the compressor and then has a negative rate of change beyond the threshold mass flow rate of gas. The threshold mass flow rate is reduced whenever the restriction is operated to reduce A_r, and the controller operates the restriction to reduce the threshold mass flow rate of gas to a value under a present mass flow rate of gas following a decrease in the present mass flow rate of gas, to thereby avoid the surge condition.

[0015] The invention also contemplates a method for controlling a compression system for an engine, where the compression system has a compressor, an inlet duct operatively connected to the compressor, and a selectively-variable restriction controlling flow through the inlet duct. The method comprises rotating the

compressor at a compressor rotational speed, reducing, by a supply into the inlet duct, a mass flow rate of gas through the compressor, in response to reducing the mass flow rate through the compressor, reducing, by the restriction, a cross- sectional area at one location of the inlet duct to thereby prevent a surge condition in the compressor.

[0016] In one aspect, the compression system further includes a restriction positioned with the inlet duct, the restriction having a cross-sectional flow area, Ar, the inlet duct having an inlet duct cross-sectional area, Ad, wherein an area ratio, ARO, is defined by A_r divided by Ad, wherein the step of reducing the cross-sectional area includes selectively reducing Ar, such that ARO varies in the range 0 < ARO < 1 .

[0017] in another aspect, the compression system further includes a controller adapted to selectively vary Ar as function of the mass flow rate of gas through the compressor.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate one or more embodiments of the invention and, together with a general description of the invention given above, and the detailed description given below, serve to explain the invention.

[0019] Fig. 1 is an exemplary centrifugal compressor performance at constant rotational speed.

[0020] Fig. 2 is schematic representation of a typical compression system.

[0021] Fig. 3 is the compression system of Fig. 2 having an inlet restriction positioned upstream of the compressor according to a first embodiment.

[0022] Fig. 4 is a graph of pressure drop of the restriction vs. velocity as a function of restriction area ratio, ARO.

[0023] Fig. 5 is a graph of pressure ratio of the restriction vs. mass flow rate as a function of restriction area ratio, ARO.

[0024] Fig. 6 is a graph of combined compressor characteristics of a theoretical restriction-compressor system as a function of compressor N_cor and restriction area ratio, ARO.

DETAILED DESCRIPTION OF THE INVENTION [0025] The present invention utilizes an inlet restriction 24 installed in the inlet duct 12 to stabilize the compression system 10. As shown in Fig. 3, the inlet restriction 24 is installed in the inlet duct 12 upstream of the compressor 14 in accordance with the principles of the invention. We discovered that coupling between the compressor 14 and inlet restriction 24 prevents a surge condition, such as mild or deep surge, and advantageously extends the low-flow operating range of the compression system 10. A mild surge condition is characterized by fluctuations in mass flow rate and pressure with the amplitudes increasing with decreasing mass flow rate. Deep surge is characterized by severe oscillations and cross-sectional averaged flow reversal with further reductions is mass flow rate. An exemplary total- to-static pressure ratio, PR_c,t_{s >} vs. mass flow rate m_c compressor characteristic 2 for the compressor 14 at a constant rotational speed is shown in Fig. 1 . The

dPRc ts/dmc = 0 line 4 is superimposed, as it provides a (conservative) estimate of the surge stability limit for an unrestricted system.

[0026] In one embodiment, the inlet restriction 24 is modeled as a simple sharp- edge orifice. While the inlet restriction 24 was modeled as an orifice, in an actual compression system 10, the inlet restriction 24 may be an orifice, a V-port segment valve, a butterfly valve, a ball valve, or any other comparable mechanism adapted to vary the cross-sectional flow area of the inlet restriction 24. Considering that the majority of pressure loss across an orifice occurs because of dissipation of kinetic energy as the flow expands from the vena contracta to the downstream duct diameter, the flow loss coefficient (as referenced to dynamic pressure in the vena contracta) of a restriction is dominated by

[0027] where A and A_d are cross-sectional flow areas of the orifice vena contracta and the downstream compressor inlet duct, respectively. The quantity AJA_d is referred to as the area ratio ARO. The area ratio ARO is less than or equal to 1 .0 and greater than 0, i.e., 0 < ARO < 1 . The total pressure drop across the restriction (location numbers [0, 1 , 2] are identified in Fig. 3)

where p_r and u_r are the fluid density and velocity at the vena contracta, respectively.

[0028] In use, the inlet restriction 24 may have an orifice with a fixed diameter and, therefore, a fixed A. Alternatively, the inlet restriction 24 may have an orifice with a selectively variable diameter such that the A may be selectively varied. As will be discussed, it is advantageous to the performance of the compressor to utilize an inlet restriction with a selectively variable diameter. To that end, the compression system 10 may further include a controller 26 operatively coupled to the inlet restriction 24. The controller 26 is adapted to selectively vary Ar as a function of mass flow rate of gas, such as air, air passing through the compressor 14. That is, the controller 26 may be programmed to operate the inlet restriction 24 to reduce Ar based on the mass flow rate of air passing through the compressor 14 to prevent a surge condition in the compressor as the mass flow rate through the compressor 14 decreases below a surge stability limit of the unrestricted (Ar = Ad or ARO = 1 ) compression system 10 at the current compressor rotational speed. In one embodiment, the controller 26 selectively varies A_r such that ARO varies from 1 to greater than 0. While air is the working fluid in this description, the concepts and benefits described herein may be applied to the compression of any gas.

[0029] A parametric study of ARO was performed to illustrate the relevant details of varying restriction, where three discrete values were selected in the range of 0.125 < ARO < 0.500. The Ap_{r tt} vs. u_r curves 40, 42, 44 for these three example restrictions are shown in Fig. 4. As expected for a fixed throat velocity, the pressure drop across the orifice increases with decreasing ARO (/^'.e., decreasing diameter of the inlet restriction). For the parametric study, the compression system was operated on a bench-top setup in a laboratory, but the invention need not rely on this exact configuration to achieve the desired benefits, e.g., prevent a surge condition. Alternatively, the compressor may be attached to any additional upstream and downstream components (for example, additional compressor stages, an internal combustion engine, or a combustor), so long as the inlet restriction is positioned upstream of the compressor.

[0030] The curves 40, 42, 44 in Fig. 4 may be alternatively represented as the ratio of total pressures downstream p_{t l} to upstream p_{t 0} of the restriction

[0031] vs. mass flow rate, as shown in Fig. 5 at 50, 52, 54. At a given flow rate, the slope dPR_{r tt}/dm_r of the restriction characteristics decreases (becomes more negative) with decreasing ARO. As ARO is reduced and the slope of the PR_r,tt vs. m_r curves 50, 52, 54 becomes increasingly negative, the pressure ratio across the orifice decreases rapidly with increasing flow rate. Therefore, the restriction must be carefully selected to ensure that the pressure drop is not greater than required.

[0032] The restriction and compressor couple in a manner similar to stages in a multi-stage turbomachine. The effective pressure ratio of this system is the product of the pressure ratios for the restriction and compressor as shown in Eqn.5.

[0033] The combined compressor characteristics 60a-60d, 62a-62d, 64a-64d of the restriction-compressor system are shown in Fig. 6, as a function of compressor corrected rotational speed and restriction area ratio. The compressor characteristic 60a, 62a, 64a for ARO = 1 in Fig. 6 represents an unrestricted compressor inlet, which is equivalent to the compressor alone characteristics 2 in Figure 1 .

[0034] The location where the slope of the combined compressor characteristics 60a-60d, 62a-62d, 64a-64d reaches zero is also included in Fig. 6 (see 66a-66d) for each of the studied ARO, which is a conservative estimate of the surge boundary (line) for the combined restriction-compressor system. Figure 6, therefore, demonstrates the significant potential of extending the low-flow operating range of compressors by configuring the inlet restriction 24 with a variable orifice whose cross-sectional area A_r may be selectively varied depending on the mass flow rate of air passing through the compressor 14.

[0035] Each of the compressor characteristic 60a-60d, 62a-62d, 64a-64d may be described for any one rotational speed of the compressor 14 as having a positive rate of change, i.e., ^{dPRc s} > 0, to a threshold mass flow rate of air through the

dm_c compressor 14 and then having a negative rate of change, i.e., ^dPRc,ts < 0, beyond

CLTH_Q

the threshold mass flow rate of air. The threshold mass flow rate is reduced whenever the inlet restriction 24 is operated to reduce Ar. The controller 26 operates the inlet restriction 24 to reduce the threshold mass flow rate of air to a value under a present mass flow rate of air following a decrease in the present mass flow rate of air, to thereby avoid the surge condition.

[0036] For all practical applications, e.g., a turbocharger on an automotive engine, it is counterproductive to restrict the compressor inlet duct more than required to shift a surge line (e.g., 66a-66d) beyond a given operating point. When the unrestricted compressor is stable (to the right of line 66a in Fig. 6), restricting the compressor inlet duct is not necessary, hence it should be eliminated. As the flow rate is reduced below the compressor characteristic peaks of the unrestricted system (to the left of line 66a in Fig. 6), however, the ARO of the restriction should be set to the maximum value that modifies the slope of the combined compressor

characteristics 60b-60d, 62b-62d, 64b-64d to a value that is stable for a given rotational speed and flow rate. The present analysis utilizes dPR_e^ _ts/dm_e^ = 0 as the target slope of the combined compressor characteristics 60a-60d, 62a-62d, 64a-64d to operate without surge. Therefore, the locus of points that stabilizes the system and minimizes compressor inlet restriction for a given N_cor passes through the restricted compressor characteristic peaks 60b-60d, 62b-62d, 64b-64d in Fig. 6 at 66b-66d.

[0037] It should be appreciated that while the above description considers an inlet restriction employed with a specific class of turbomachinery (automotive

turbocharger centrifugal compressor, for example), the inlet restriction is also applicable for use on other turbomachine compressors, such as, centrifugal (or radial) and axial compressors. The inlet restriction may be used with single-stage compressors, multi-stage compressors (in series), parallel configurations, or a combination of series and parallel configurations.

[0038] While the inlet restriction 24 and compressor 14 are illustrated and described herein as separate components, the restriction and compressor may be combined into a single piece of hardware and still achieve the desired benefits, e.g., prevent a surge condition. In other words, a restriction and a compressor may be integrated into a single part so long as the restriction is upstream of a compressor stage.

[0039] While the invention has been illustrated by a description of various embodiments, and while these embodiments have been described in considerable detail, it is not the intention of the Applicant to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. The invention in its broader aspects is therefore not limited to the specific details, representative apparatus and method, and illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the spirit or scope of the Applicant’s general inventive concept.

WHAT IS CLAIMED IS:

Claims

WHAT IS CLAIMED IS:

1. A compression system (10) for an engine comprising:

a compressor (14) having a compressor inlet and a compressor outlet;

an inlet duct (12) operatively connected to the compressor inlet, the inlet duct having an inlet duct cross-sectional area, Ad;

an outlet duct (16) operatively connected to the compressor outlet; and a restriction (24) positioned within the inlet duct, the restriction having a cross- sectional flow area, A_r, wherein A_r is equal to or less than Ad.

2. The compression system (10) of claim 1 , wherein an area ratio, ARO, is defined by Ar divided by Ad, Ar being selectively variable such that ARO is variable in the range 0 < ARO < 1.

3. The compression system (10) of claim 1 , wherein A_r is selectively variable, the compression system (10) further comprising a controller (26) adapted to selectively vary Ar as a function of a mass flow rate of gas passing through the compressor (14).

4. The compression system (10) of claim 3, wherein an area ratio, ARO, is defined by Ar divided by Ad, the controller (26) selectively varying A_r such that ARO varies in the range 0 < ARO < 1.

5. The compression system (10) of claim 1 or claim 2, further comprising: a controller (26) adapted to selectively vary A_r as a function of a mass flow rate of gas passing through the compressor (14);

wherein the controller (26) is programmed to operate the restriction to reduce Ar based on the mass flow rate of gas passing through the compressor (14) to prevent a surge condition in the compressor (14) as the mass flow rate of gas through the compressor (14) decreases below a surge stability limit defined at Ar = Ad at a current compressor rotational speed, and

wherein the surge condition is characterized by a fluctuation in the mass flow rate of gas through the compressor (14).

6. The compression system (10) of any of the preceding claims, further comprising a throttle duct (20) operatively connected to the outlet duct (16) and having a throttle mechanism (22) disposed within the throttle duct (20).

7. A compression system (10) for an engine, comprising:

a compressor (14) having a compressor inlet;

an inlet duct (12) operatively connected to the compressor inlet, the inlet duct (12) having an inlet duct cross-sectional area, Ad;

a restriction (24) positioned within the inlet duct (12), the restriction (24) having a cross-sectional flow area, Ar, which is equal to or less than Ad and is selectively variable; and a controller (26) adapted to selectively vary A_r as a function of a mass flow rate of gas passing through the compressor (14);

wherein the controller (26) is programmed to operate the restriction (24) to reduce Ar based on the mass flow rate of gas passing through the compressor (14) to prevent a surge condition in the compressor (14) as the mass flow rate of gas through the compressor (14) decreases below a surge stability limit defined at Ar = Ad at a current compressor rotational speed, and

8. The compression system (10) of claim 7 wherein an area ratio, ARO, is defined by Ar divided by Ad, the controller (26) selectively varying A_r such that ARO varies in the range 0 < ARO < 1.

9. The compression system (10) of claim 7 or claim 8, wherein the compressor (14) defines a compressor characteristic of total-to-static pressure ratio, PR_c,t_s , versus mass flow rate, m_c, for a constant rotational speed of the compressor (14), wherein the cLPR_{c ts} compressor (14) will encounter the surge condition when a rate of change,— r~, of the

compressor characteristic is positive.

10. The compression system (10) of claim 9 wherein the compressor characteristic for any one rotational speed of the compressor (14) has positive rate of change to a threshold mass flow rate of gas through the compressor (14) and then has a negative rate of change beyond the threshold mass flow rate of gas, the threshold mass flow rate is reduced whenever the restriction is operated to reduce A_r, and the controller (26) operates the restriction (24) to reduce the threshold mass flow rate of gas to a value under a present mass flow rate of gas following a decrease in the present mass flow rate of gas, to thereby avoid the surge condition.

11. A method for controlling a compression system (10) for an engine, the

compression system (10) having a compressor (14), an inlet duct (12) operatively connected to the compressor (14), and a selectively-variable restriction (24) controlling flow through the inlet duct (12), the method comprising:

rotating the compressor (14) at a compressor rotational speed;

reducing, by a supply into the inlet duct (12), a mass flow rate of gas through the compressor (14); and

in response to reducing the mass flow rate through the compressor (14), reducing, by the restriction, a cross-sectional area at one location of the inlet duct (12) to thereby prevent a surge condition in the compressor (14).

12. The method of claim 11 wherein the compression system (10) further includes a restriction (24) positioned with the inlet duct (12), the restriction having a cross-sectional flow area, A_r, the inlet duct (12) having an inlet duct cross-sectional area, Ad, wherein an area ratio, ARO, is defined by Ar divided by Ad, wherein the step of reducing the cross- sectional area includes selectively reducing Ar, such that ARO varies in the range 0 < ARO < 1.

13. The method of claim 12 wherein the compression system (10) further includes a controller (26) adapted to selectively vary A_r as function of the mass flow rate of gas through the compressor (14).