# Patent

## Title
Mixing device for a gas burner

## Abstract

A mixing device (10) for a gas burner, with a housing (11) having a combustion air inlet (13), a gas inlet (14) and an outlet (15) for the mixture of gas and combustion air, and with a Venturi device (12) which is positioned in the housing, wherein the Venturi device is contoured to form a contraction section (19), a mixing section (20) and a diffuser section (21). A guide device (22) for combustion air, which divides combustion air which enters the mixing device via the combustion air inlet (13) into a primary flow and a secondary flow, may be positioned inside the Venturi device, wherein the combustion air of the primary flow, issuing from the combustion air inlet, can be fed to the mixing section (20) via the contraction section (19), and wherein the combustion air of the secondary flow, issuing from the combustion air inlet, can be mixed downstream of the mixing section (20) in the region of the diffuser section (21) with the mixture of the gas and the combustion air of the primary flow.

## Background
The present application claims priority to German Patent Application No. DE 10 2010 010 791.3, filed on Mar. 9, 2010, entitled “MIXING DEVICE FOR A GAS BURNER”, which is incorporated herein by reference.
TECHNICAL FIELD
This disclosure relates to a mixing device for a gas burner.
BACKGROUND
A mixing device for a gas burner with a housing and a Venturi device which is positioned in the housing is known from DE 296 17 621 U1. The housing of the mixing device which is disclosed there has a combustion air inlet, a gas inlet and an outlet for the mixture of gas and combustion air. The Venturi device which is positioned in the housing is constructed as a Venturi nozzle and serves for the mixing of the combustion air and the gas.
A Venturi device, designed as a Venturi nozzle, for a mixing device of a gas burner, which is positioned in a housing of a mixing device and formed from an inlet funnel and a discharge funnel, is known from DE 197 43 464 C1, wherein the Venturi device, specifically the inlet funnel and the discharge funnel of the Venturi device, is contoured to form a contraction section, a mixing section and a diffuser section, and in such a way that the combustion air, issuing from the combustion air inlet, can be fed to the mixing section via the contraction section which narrows in the direction of the mixing section, and that the mixture of gas and combustion air, issuing from the mixing section, can be fed to the outlet via the widening diffuser section. In the mixing section, an inlet-side section of the discharge funnel overlaps a discharge-side section of the inlet funnel, forming an annular gap or an annular gap-like nozzle, via which the gas can be admixed with the combustion air.
For further clarification of the mixing devices of a gas burner, which are known from the prior art, reference may be made to FIG. 1 which shows a schematized cross section of a mixing device 10′ which is known from the prior art. The mixing device 10′ according to FIG. 1 comprises a housing 11′, inside which a Venturi device 12′ is positioned. The housing 11′ of the mixing device 10′ has a combustion air inlet 13′ for combustion air V, a gas inlet 14′ for gas G and also an outlet 15′ for the mixture M of gas and combustion air. The Venturi device 12′, which is positioned in the housing 11′, also has the combustion air inlet 13′ and also the outlet 15′ for the mixture M of gas and combustion air, wherein the gas G, which enters the housing 11′ via the gas inlet 14′, flows radially outward around the Venturi device 12′ and, via an annular gap 16′ which is formed between an inlet funnel 17′ and a discharge funnel 18′ of the Venturi device 12′, can be admixed with or added to the combustion air V. The Venturi device 12′, which includes the inlet funnel 17′ and the discharge funnel 18′, forming a contraction section 19′, a mixing section 20′ and a diffuser section 21′, is contoured in such a way that the combustion air V, issuing from the combustion air inlet 13′ of the housing 10′ or of the Venturi device 12′, can be fed to the mixing section 20′ via the contraction section 19′ which narrows in the direction of the mixing section 20′. The mixture M of gas and combustion air, issuing from the mixing section 20′, can be fed to the outlet 15′ via the diffuser section 21′, which widens in the direction of the outlet 15′. The annular gap 16′ is located in the mixing section 20′. The annular gap 16′ has a cross section which results from a diameter difference between the outside diameter of a discharge-side end of the inlet funnel 17′ and the inside diameter of an inlet-side end of the discharge funnel 18′ in the mixing section 20′. The outside diameter of the annular gap 16′ thus corresponds to the inside diameter of the inlet-side end of the discharge funnel 18′ and therefore to the diameter of the mixing section 20′.
A fan or a blower is typically associated with the outlet 15′ or with the combustion air inlet 13′ of such a mixing device of a gas burner, wherein a speed of the fan or of the blower determines the quantity or the volumetric flow of the mixture of gas and combustion air which is fed to the gas burner. The ratio of gas and combustion air in the combustion air/gas mixture is meant to be relatively constant and is primarily determined by a ratio of the cross section of the inlet funnel 17′ in the mixing section 20′ of the Venturi device 12′ and the cross section of the annular gap 17′ which is formed between the inlet funnel 17′ and the discharge funnel 18′ of the Venturi device 12′. Regardless of the speed of the fan, the ratio of gas and combustion air in the combustion air/gas mixture is supposed to be constant, wherein with increasingly or reducing speed of the fan, undesirable deviations in the ratio of gas and combustion air develop on account of inaccuracies with respect to control engineering. In the case of known mixing devices, the speed of a fan which interacts with the mixing device can be reduced only to a certain extent while maintaining the desired ratio of gas and combustion air. This ultimately results in a realizable modulation range of between 1 and 5, in which a high modulation quality, specifically a desired ratio of gas and combustion air in the combustion air/gas mixture, can be provided. A modulation of 1 corresponds to a full load speed of the fan and a modulation of 5 corresponds to 20% of the full load speed of the fan. With mixing devices which are known from the prior art, the speed of the fan which interacts with the mixing device can therefore be reduced only to 20% of the full load speed of the fan while maintaining the desired ratio of gas and combustion air.
SUMMARY
This disclosure relates to a new type of mixing device for a gas burner, which on the one hand has a low flow resistance and on the other hand enables a broader modulation range while helping to ensure a high modulation quality.
In one illustrative embodiment, a guide device for combustion air, which divides combustion air which enters the mixing device via the combustion air inlet into a primary flow and a secondary flow, is positioned inside the Venturi device, wherein the combustion air of the primary flow, issuing from the combustion air inlet, can be fed to the mixing section via the contraction section, and wherein the combustion air of the secondary flow, issuing from the combustion air inlet, can be mixed downstream of the mixing section in the region of the diffuser section with the mixture of the gas and the combustion air of the primary flow.
In some instances, it may be ultimately possible to have a broader modulation range between 1 and 10 with good modulation quality for a fan which interacts with the mixing device. Using the mixing device according to this disclosure, the speed of a fan which interacts with the mixing device may be reduced to 10% (or less) of the full load speed of the fan while maintaining the desired ratio of gas and combustion air. In some instances, the speed of a fan which interacts with the device may be reduced to a greater extent than in the case of mixing devices which are known from the prior art, and this while maintaining a desired ratio of gas and combustion air. Furthermore, a low flow resistance may be accomplished.
The above summary is not intended to describe each disclosed embodiment or every implementation. The Figures, Description and Examples which follow more particularly exemplify these embodiments.



BRIEF DESCRIPTION
The disclosure may be more completely understood in consideration of the following description of various embodiments in connection with the accompanying drawings, in which:
FIG. 1 shows a schematized cross section through a mixing device which is known from the prior art for mixing gas and combustion air for a gas burner;
FIG. 2 shows a schematized cross section through a mixing device for mixing gas and combustion air for a gas burner according to an exemplary embodiment; and
FIG. 3 shows a schematized cross section through a mixing device for mixing gas and combustion air for a gas burner according to another exemplary embodiment.



DESCRIPTION
FIG. 2 shows a schematized cross section of a first exemplary embodiment of a mixing device 10, wherein the mixing device 10 includes a housing 11, inside which a Venturi device 12 is positioned. The housing 11 of the mixing device 10 has a combustion air inlet 13 for combustion air V, a gas inlet 14 for gas G, and also an outlet 15 for the mixture M of gas and combustion air.
The Venturi device 12, which is positioned in the housing 11, also has the combustion air inlet 13 and also the outlet 15 for the mixture of gas and combustion air, wherein the gas G, which enters the housing 11 via the gas inlet 14, flows radially outwards around the Venturi device 12, which is visible in FIG. 2, and, via an annular gap 16 which is formed between an inlet funnel 17 and a discharge funnel 18 of the Venturi device 12, can be admixed with or added to the combustion air V.
The Venturi device 12, which includes the inlet funnel 17 and the discharge funnel 18, is contoured to form a contraction section 19, a mixing section 20 and a diffuser section 21.
In some instances, a guide device 22 for the combustion air is positioned inside the Venturi device 12. The guide device 22 divides the combustion air V, which enters the mixing device 10 or the Venturi device 12 via the combustion air inlet 13, into a primary flow V1 and a secondary flow V2. The combustion air of the primary flow V1, which flows around the guide device 22, issuing from the combustion air inlet 13, can be fed to the mixing section 20 via the contraction section 19, wherein the combustion air of the primary flow V1 is mixed in the region of the mixing section 20 with the gas G which enters the mixing device 10 via the gas inlet 14. The combustion air of the secondary flow V2, which flows through the guide device 22, issuing from the combustion air inlet 13, can be mixed downstream of the mixing section 20 in the region of the diffuser section 21 with the mixture of the combustion air of the primary flow V1 and the gas.
By establishing the secondary flow V2 of the combustion air, which flows through the guide device 22 and is mixed with the gas and the combustion air of the primary flow V1 downstream of the mixing section 20, a broader modulation range can be realized, and this while maintaining a high modulation quality.
In the exemplary embodiment of the mixing device 10 which is shown in FIG. 2, the guide device 22 is positioned centrally in the Venturi device 12, and in some cases concentrically. Also, the guide device 22 has a tubular contour, specifically a continuously cylindrical contour, in the exemplary embodiment which is shown in FIG. 2, and has a defined diameter d22 and a defined length l22. In the depicted exemplary embodiment of FIG. 2, the diameter d22 of the tubular guide device 22 is constant over the entire length l22 of the device.
The inlet 13 for the combustion air V has a diameter d13 and the outlet 15 for the mixture M of gas and combustion air has a diameter d15. The mixing section 20 has a diameter d20.
The ratio d13/d15 between the diameter d13 of the inlet 13 for the combustion air V and the diameter d15 of the outlet 15 for the mixture M of gas and combustion air may be, for example, between 0.75 and 1.25, especially (100±25) %. This ratio d13/d15 is preferably 1.0.
The ratio d20/d15 between the diameter d20 of the mixing section 20 and the diameter d15 of the outlet 15 for the mixture M of gas and combustion air may be, for example, between 0.25 and 0.75, especially (50±25) %. This ratio d20/d15 is preferably 0.5.
The ratio l19/d20 between the length l19 of the contraction section 19 and the diameter d20 of the mixing section 20 may be, for example, between 0.5 and 1.5, especially 1±0.5. This ratio l19/d20 is preferably 1.
The ratio l21/d20 between the length l21 of the diffuser section 21 and the diameter d20 of the mixing section 20 may be, for example, between 2 and 6, especially 4±2. This ratio l21/d20 is preferably 4.
As already mentioned, the guide device 22, which may be positioned in the Venturi device 12, has the diameter d22. The ratio d22/d20 between the diameter d22 of the guide device 22 and the diameter d20 of the mixing section 20 may be, for example, between 0.05 and 0.55, especially (30±25) %. This ratio d22/d20 is preferably 0.3.
The ratio l22/d22 between the length l22 of the guide device 22 and the diameter d22 of the guide device may be, for example, between 5 and 15, especially 10±5. This ratio l22/d22 is preferably 10.
The distance x between the flow discharge-side end 24 of the guide device 22 and the outlet 15 for the mixture M of gas and combustion air is dimensioned in such a way that the ratio x/d20 between this distance x and the diameter d20 of the mixing section 20 may be between 0 and 2, especially 1±1. The ratio x/d20 is preferably 1.
In the depicted exemplary embodiment of FIG. 2, the flow discharge-side end 24 of the guide device 22 may lie upstream of the outlet 15 for the mixture of gas and combustion air.
The flow discharge-side end 24 of the guide device 22, which in the depicted exemplary embodiment of FIG. 2 preferably has a cylindrical contour, can be flared like a funnel.
The flow inlet-side end 23 of the guide device 22 in the depicted exemplary embodiment of FIG. 2 may lie downstream of the combustion air inlet 13. The offset between the flow inlet-side end 23 of the guide device 22 and the combustion air inlet 13 preferably corresponds at most to the diameter d22 of the guide device 22.
In contrast to this, it is also possible, however, that the flow inlet-side end 23 of the guide device 22 terminates flush with the combustion air inlet 13, or may even extend out past the air inlet 13.
In the exemplary embodiment of FIG. 2, the tubular guide device 22 is shown to be cylindrically contoured, and therefore has a diameter d22 which is constant over the entire length l22 of the guide device 22.
In the exemplary embodiment of FIG. 2, the guide device 22 has a circular contour in cross section. In contrast to this, the guide device 22 may have an oval or elliptical contour in cross section, wherein the diameter d22 is then the so-called large axis of the respective ellipse. These are only examples, and it is contemplated that the guide device 22 may have any suitably shaped contour, as desired.
FIG. 3 shows another exemplary embodiment of a mixing device 110, wherein the mixing device 110 includes a housing 111, inside which a Venturi device 112 is positioned. The housing 111 of the mixing device 110 has a combustion air inlet 113 for combustion air V, a gas inlet 114 for gas G and also an outlet 115 for the mixture M of gas and combustion air. The Venturi device 112, which is positioned in the housing 111, also has the combustion air inlet 113 and also the outlet 115 for the mixture of gas and combustion air, wherein the gas G, which enters the housing 111 via the gas inlet 114, flows radially outward around the Venturi device 112 which is visible in FIG. 3, and, via an annular gap 116 which is formed between an inlet funnel 117 and a discharge funnel 118 of the Venturi device 112, can be admixed with or added to the combustion air V. In the illustrative embodiment, the Venturi device 112, which may include the inlet funnel 117 and the discharge funnel 118, is contoured to form a contraction section 119, a mixing section 120 and a diffuser section 121.
As show in FIG. 3, a guide device 122 for the combustion air may be positioned inside the Venturi device 112. The guide device 122 may divide the combustion air V, which enters the mixing device 110 or the Venturi device 112 via the combustion air inlet 113, into a primary flow V1 and a secondary flow V2. The combustion air of the primary flow V1, which flows around the guide device 122, issuing from the combustion air inlet 113, can be fed to the mixing section 120 via the contraction section 119, wherein the combustion air of the primary flow V1 is mixed in the region of the mixing section 120 with the gas G which enters the mixing device 110 via the gas inlet 114.
The combustion air of the secondary flow V2, which flows through the guide device 122, issuing from the combustion air inlet 113, can be mixed downstream of the mixing section 120 in the region of the diffuser section 121 with the mixture of the combustion air of the primary flow V1 and the gas. By establishing the secondary flow V2 of combustion air which flows through the guide device 122 and is mixed downstream of the mixing section 120 with the gas and the combustion air of the primary flow V1, a broad modulation range can be realized, and this while maintaining a high modulation quality.
In the exemplary embodiment of the mixing device 110 which is shown in FIG. 3, the guide device 122 may have a tubular contour and has two sections, specifically a truncated cone-like section 104, which widens out in the flow direction, and a cylindrical section 105, which adjoins the truncated cone-like section 104 downstream. The truncated cone-like section 104 has a length l104, a flow inlet-side or upstream diameter d102 and a flow discharge-side or downstream diameter d101. The cylindrical section 105 has a length l105 and a diameter which corresponds to the flow discharge-side or downstream diameter d101 of the truncated cone-like section 104. The inlet 113 for the combustion air V has a diameter d103 and the mixing section 120 has a diameter d100.
A ratio l104/d102 between the length l104 of the truncated cone-like section 104 of the guide device 122 and the flow inlet-side or upstream diameter d102 of the truncated cone-like section 104 of the guide device 122 may be, for example, between 2 and 6, especially 4±2. The ratio l104/d102 is preferably 4.
A ratio l105/d102 between the length l105 of the cylindrical section 105 of the guide device 122 and the flow inlet-side or upstream diameter d102 of the truncated cone-like section 104 of the guide device 122 may be, for example, between 1 and 3, especially 2±1. The ratio l105/d102 is preferably 2.
A ratio d102/d101 between the flow inlet-side or upstream diameter d102 of the truncated cone-like section 104 of the guide device 122 and the flow discharge-side or downstream diameter d101 of the truncated cone-like section 104 of the guide device 122 may be, for example, between 0.25 and 0.75, especially (50±25) %. This ratio d102/d101 is preferably 0.5.
A ratio d101/d100 between the flow discharge-side or downstream diameter d101 of the section 104 of the guide device 122, which corresponds to the diameter of the section 105, and the diameter d100 of the mixing section 120 may be, for example, between 0.81 and 0.99, especially (90±9) %. This ratio d101/d100 is preferably 0.9.
A ratio d103/d100 between the diameter d103 of the inlet 113 for the combustion air V and the diameter d100 of the mixing section 120 may be, for example, between 0.9 and 1.3, especially (110±20) %. This ratio d103/d100 is preferably 1.1.
A ratio y/d100 of the distance y between a flow discharge-side end 124 of the guide device 122 and the outlet 115 for the mixture of gas and combustion air and the diameter d100 of the mixing section 120 may be, for example, between 0.5 and 2.5.
In the exemplary embodiment of FIG. 3, the flow discharge-side end 124 of the guide device 122 lies upstream of the outlet 115 for the mixture M, specifically exactly at the end of the mixing section 120 according to FIG. 3, however, this is not required.
The flow discharge-side end 124 of the guide device 122 may terminate flush with the end of the mixing section 120 according to FIG. 3. The flow discharge-side end 124 of the guide device 122 can also lie downstream (or even upstream in some cases) of the end of the mixing section 120.
In each case, the combustion air of the secondary flow V2, issuing from the combustion air inlet 113, may be mixed downstream of the mixing section 120 in the region of the diffuser section 121 with the mixture of the gas G and the combustion air of the primary flow V1.
The flow inlet-side end 123 of the guide device 122 may lie exactly at the inlet 113 of the combustion air and terminate flush with the combustion air inlet 113, as shown in exemplary embodiment of FIG. 3, but this is not required. It is also possible that the flow inlet-side end 123 of the guide device 122 lies downstream (or even upstream) of the combustion air inlet 113.
The flow discharge-side end of the section 104 of the guide device 122, and therefore the flow inlet-side end of the section 105 of the guide device 122, may terminate flush with the start of the mixing section 120 according to FIG. 3, but again, this is not required. For example, the flow discharge-side end of the section 104 of the guide device 122, and therefore the flow inlet-side end of the section 105 of the guide device 122, can lie downstream or upstream of the start of the mixing section 120.
In the exemplary embodiment of FIG. 3, the guide device 122 has a circular contour in cross section. In contrast to this, it is also possible that the guide device 122 has an oval, elliptical, or any other suitable shape contour in cross section.
In the exemplary embodiment of FIG. 2, the guide device 22 provides a type of bypass to the contraction section 19 in which the secondary flow V2 is neither contracted nor expanded. In the exemplary embodiment of FIG. 3, on the other hand, the secondary flow V2 is expanded in the section 104 of the guide device 122, whereas in parallel to this the primary flow V1 is contracted in the contraction section 119. With both exemplary embodiments, a broader modulation range can be realized, and this while maintaining a high modulation quality. At the same time, a low flow resistance or throughflow resistance may be achieved.
LIST OF DESIGNATIONS


    10, 110, 10′ Mixing device
    11, 111, 11′ Housing
    12, 112, 12′ Venturi device
    13, 113, 13′ Combustion air inlet
    14, 114, 14′ Gas inlet
    15, 115, 15′ Outlet
    16, 116, 16′ Annular gap
    17, 117, 17′ Inlet funnel
    18, 118, 18′ Discharge funnel
    19, 119, 19′ Contraction section
    20, 120, 20′ Mixing section
    21, 121, 21′ Diffuser section
    22, 122 Guide device
    23, 123 Flow inlet-side end
    24, 124 Flow discharge-side end
    104 Section
    105 Section
    V Combustion air
    V1 Primary flow
    V2 Secondary flow
    G Gas
    M Mixture of gas and combustion air

## Claims

1. A mixing device for mixing gas and combustion air for a gas burner, the mixing device having a housing that defines a combustion air inlet, a gas inlet, and an outlet, the mixing device comprising:
a venturi which serves to mix the combustion air and the gas, wherein the venturi includes a contraction section, a mixing section and a diffuser section; and
a guide device positioned inside the venture, the guide device divides combustion air which enters the mixing device via the combustion air inlet of the housing into a primary flow and a secondary flow, wherein the combustion air of the primary flow, issuing from the combustion air inlet of the housing, is fed to the mixing section via the contraction section and is mixed with the gas to form a first mixture, and wherein the combustion air of the secondary flow, issuing from the combustion air inlet of the housing, is mixed downstream of the mixing section in the region of the diffuser section with the mixture of the gas and the combustion air of the primary flow to form a second mixture.

2. The mixing device of claim 1, wherein the guide device is positioned concentrically in the venturi.

3. The mixing device of claim 1, wherein the guide device has a cylindrical shape.

4. The mixing device of claim 3, wherein a ratio between a diameter of the guide device and a diameter of the mixing section of the venturi is between 0.05 and 0.55.

5. The mixing device of claim 3, wherein a ratio between a length of the guide device and a diameter of the guide device is between 5 and 15.

6. The mixing device of claim 3, wherein a ratio between a length of the contraction section of the venturi and a diameter of the mixing section of the venturi is between 0.5 and 1.5.

7. The mixing device of claim 3, wherein a ratio between a length of the diffuser section of the venturi and a diameter of the mixing section of the venturi is between 2 and 6.

8. The mixing device of claim 3, wherein a ratio of a distance (x) between a flow discharge-side end of the guide device and the outlet of the housing, and a diameter of the mixing section of the venturi is between 0 and 2.

9. The mixing device of claim 3, wherein:
a ratio between a diameter of the guide device and a diameter of the mixing section of the venturi is between 0.05 and 0.55;
a ratio between a length of the guide device and the diameter of the guide device is between 5 and 15;
a ratio between a length of the contraction section of the venturi and the diameter of the mixing section of the venturi is between 0.5 and 1.5;
a ratio between a length of the diffuser section of the venturi and the diameter of the mixing section of the venturi is between 2 and 6.

10. The mixing device of claim 9, wherein a ratio of a distance (x) between a flow discharge-side end of the guide device and the outlet of the housing, and the diameter of the mixing section of the venturi is between 0 and 2.

11. The mixing device of claim 1, wherein the guide device is of a tubular design, and includes a truncated cone-shaped section which widens out in the flow direction and a cylindrical section which adjoins the truncated cone-shaped section downstream.

12. The mixing device of claim 11, wherein a ratio between a length of the truncated cone-shaped section of the guide device and an upstream diameter at an upstream end of the truncated cone-shaped section of the guide device is between 2 and 6.

13. The mixing device of claim 11, wherein a ratio between a length of the cylindrical section of the guide device and an upstream diameter at an upstream end of the truncated cone-shaped section of the guide device is between 1 and 3.

14. The mixing device of claim 11, wherein a ratio between an upstream diameter at an upstream end of the truncated cone-shaped section of the guide device and a downstream diameter at a downstream end of the truncated cone-shaped section of the guide device is between 0.25 and 0.75.

15. The mixing device of claim 11, wherein a ratio between a downstream diameter at a downstream end of the truncated cone-shaped section of the guide device and a diameter of the mixing section of the venturi is between 0.81 and 0.99.

16. The mixing device of claim 11, wherein a ratio of a distance (y) between a flow discharge-side end of the guide device and the outlet of the housing, and a diameter of the mixing section of the venturi is between 0.5 and 2.5.

17. The mixing device of claim 11, wherein:
a ratio between a length of the truncated cone-shaped section of the guide device and an upstream diameter at the upstream end of the truncated cone-shaped section of the guide device is between 2 and 6;
a ratio between a length of the cylindrical section of the guide device and the upstream diameter at the upstream end of the truncated cone-shaped section of the guide device is between 1 and 3;
a ratio between the upstream diameter at the upstream end of the truncated cone-shaped section of the guide device and a downstream diameter at a downstream end of the truncated cone-shaped section of the guide device is between 0.25 and 0.75; and
a ratio between the downstream diameter at the downstream end of the truncated cone-shaped section of the guide device and a diameter of the mixing section of the venturi is between 0.81 and 0.99.

18. The mixing device of claim 17, wherein a ratio of a distance (y) between a flow discharge-side end of the guide device and the outlet of the housing, and the diameter of the mixing section of the venturi is between 0.5 and 2.5.

19. A mixing device for mixing gas and combustion air for a gas burner, the mixing device having a housing that defines a combustion air inlet, a gas inlet, and an outlet, the mixing device comprising:
a venturi that includes a contraction section, followed by a mixing section, followed by a diffuser section;
the gas inlet of the mixing device is in fluid communication with the mixing section of the venturi to provide gas to the mixing section of the venturi;
a guide device having an outer wall that defines a lumen that extends through the guide device from an upstream end of the guide device to a downstream end of the guide device, wherein the guide device is positioned in the venturi and passes through at least part of the mixing section of the venture such that an outlet of the guide device is downstream of an inlet of the mixing section; and
the guide device dividing combustion air that enters the combustion air inlet of the mixing device into a primary flow, which flows between the outer wall of the guide device and an inner wall of the venturi, and a secondary flow, which flows through the lumen of the guide device.

20. A mixing device for mixing gas and combustion air for a gas burner, the mixing device having a housing that defines a combustion air inlet, a gas inlet, and an outlet, the mixing device comprising:
a venturi that includes a contraction section, followed by a mixing section, followed by a diffuser section;
the gas inlet of the mixing device is in fluid communication with the mixing section of the venturi to provide gas to the mixing section of the venturi;
a guide device having an outer wall that defines a lumen that extends through the guide device from an upstream end of the guide device to a downstream end of the guide device, wherein the upstream end of the guide device is positioned upstream of the mixing section of the venturi and the downstream end of the guide device is downstream of the mixing section of the venturi; and
the guide device dividing combustion air that enters the combustion air inlet of the mixing device into a primary flow, which flows between the outer wall of the guide device and an inner wall of the venturi, and a secondary flow, which flows through the lumen of the guide device.