JPS63207913A - Fluid heating device with radiator - Google Patents

Abstract

PURPOSE:To effectively transfer combustion heat without burning fire combustion means and a heat exchange part to prevent incomplete combustion by providing a plurality of gas-permeable radiators disposed by inclining a first heat transfer tube group with respect to the flowing direction of a combustion gas, and a second heat transfer group disposed at the downstream in the vicinity of a first heat transfer tube group. CONSTITUTION:Gas-permeable radiators 20 are disposed in parallel to heat transfer tube 19a and 19b and slantly with respect to the flowing direction of a combustion gas between respective heat transfer tubes 19a and 19b. The radiators 20 radiate radiant heat to a second heat transfer tube group as well as a first heat transfer tube group 10 positioned at both sides of each radiator 20. As the radiator 20 a honeycomb plate is used. Since the honeycomb plate has a tendency to have a large difference in temperature distribution between a part near to the heat transfer tube and a part far therefrom and further a large temperature difference is imparted at the time of ignition and at the time of fire-extinguishment, the radiator is made from ceramics having excellent heat resistance and thermal shock resistance. At the upper part of in the vicinity of the first heat transfer tube group 19 and the radiators 20, the second heat transfer tube group with fins 21 is disposed in a zigzag manner.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field] The present invention relates to a fluid heating device used in water heaters, bathtubs, hot water boilers, and the like.

[Prior art and its problems] Conventionally, in fluid heating devices such as water heaters, bathtubs, and hot water boilers, fuel is combusted in a combustion chamber downstream of a burner, and then the combustion gas is passed between a group of heat transfer tubes. It was designed to heat fluids such as water flowing inside a group of heat transfer tubes, mainly by using convective heat transfer.

In recent years, in order to make these fluid heating devices as compact as possible, the combustion chamber has been made as small as possible, and
This tends to increase the amount of heat transferred per unit volume of the heat exchange section.

By the way, if the combustion chamber is simply downsized, the flame will extend to the heat exchanger without completing the combustion reaction within the combustion chamber, and as a result, the fuel in the middle of the combustion reaction will come into contact with the walls of the heat transfer tubes and cool the flame. This sometimes caused the combustion reaction to stop, resulting in incomplete #A firing. This not only results in a loss of fuel, but also generates carbon oxide, soot, aldehyde, etc., which has an adverse effect on the human body.

In addition, if the combustion chamber is downsized and integrated with the heat exchange section, if unburned fuel components adhere to and accumulate on the surface of the heat transfer tube, the heat transfer tube may be overheated and damaged, or the combustion gas in the combustion chamber may not mix well. As a result, the temperature distribution tends to become large, and the heat transfer load increases locally, which can also damage the heat transfer tubes.

For this reason, there is a limit to the small size of the combustion chamber (II); for example, in currently commercially available gas water heaters, the size difference between the combustion chamber and the heat exchange section is approximately 2 = 1, and the size difference between the burner tip and the The distance to the downstream heat exchanger tube is 20 to 30 cm, and the combustion chamber load was kept to less than 5x 10' kcal/m3/hr.The combustion chamber could be made smaller to bring the heat exchanger closer to the flame. This has been difficult because it accelerates damage to the heat exchanger tubes and increases CO generation.

As long as convective heat transfer is used, in order to increase the amount of heat transfer per unit volume of the heat exchanger, the distance between plate fins should be narrowed and the number of plate fins should be increased to increase the heat transfer area per unit volume. Measures can be taken to increase the heat transfer coefficient by increasing the flow rate of the combustion gas, or by increasing the temperature of the combustion gas flowing into the heat exchanger to increase the temperature difference between the heating side and the heated side. It becomes necessary.

However, in order to prevent clogging between the plate fins, the lower limit of the plate fin interval is about 2.6 mm. In order to suppress pressure loss, the upper limit of the combustion gas flow velocity is about 10 m/s. Roughly speaking, if the temperature of the combustion gas flowing into the heat exchange section is increased, the temperature at the tip of the plate fin will rise, causing problems with heat resistance and durability, so the upper limit of the combustion gas inflow temperature is about 1000℃. . Thus, there was a limit to the miniaturization of the heat exchange section.

"Objective of the Invention" The object of the present invention is to solve the problems of the prior art described above, to effectively transfer combustion heat without burning out the combustion means and the heat exchanger, and to suppress incomplete combustion. However, it is an object of the present invention to provide a fluid heating device that can reduce the size of the entire device.

"Structure of the Invention" The fluid heating device of the present invention includes a combustion means, a first heat exchanger tube group disposed adjacent to and downstream of the combustion means, and a combustion gas in the darkness of the heat exchanger tubes constituting the first heat exchanger tube group. The heat exchanger tube is characterized by comprising a plurality of air permeable radiators arranged at an angle with respect to the flow direction of the heat exchanger, and a second heat exchanger tube group arranged adjacent to and downstream of the first heat exchanger tube group.

In the present invention, gaseous fuels such as city gas, buoyant gas, and natural gas, or vaporized liquid fuels such as kerosene can be used as the fuel. Combustion means include a diffusion combustion type burner that supplies combustion air and fuel separately to the combustion chamber, or a premixed burner that mixes combustion air and fuel in the required ratio and supplies it to the combustion chamber. Burners etc. are used. A preferred example of the premix burner is one having a planar burner plate.

In the first heat exchanger tube group, the heat exchanger tubes constituting the group are usually arranged parallel to each other and perpendicular to the flow direction of the combustion gas.
! and are preferably arranged in multiple stages, particularly in a staggered manner 1c'
and generally located downstream of and proximate to the combustion means.

The first heat exchanger tube group is arranged such that the end of the heat exchanger tubes or the fins attached to the first heat exchanger tube group is placed closest to the combustion means, for example, in the flame formed by the combustion means.
Alternatively, it is placed close to the tip of the flame. Specifically, it is preferable that the distance between the fuel gas discharge port of the combustion means (for example, the outlet side surface of the burner plate) and the upstream line of the heat exchanger tube is 5 to 50 mm. In other words,
The length of the flame varies depending on the design of the combustion means, but is generally about 5 to 50 mm, so the most upstream heat exchanger tube in the first heat exchanger tube group is ultimately placed near the tip of the flame. Become.

If the first heat transfer tube group is placed at a position farther from the combustion means than the above position, the temperature of the combustion gas will decrease due to heat loss or cooling pipes in the casing surrounding the combustion chamber.
There is a possibility that the effects of the present invention cannot be sufficiently obtained, or the gas thickness increases and the radiant heat input from the high temperature combustion gas to the burner increases, leading to damage to the burner and backfire. Conversely, when arranging the first heat exchanger tube group closer to the combustion means than the above position, it is preferable to take care not to increase CO generation.

The first heat transfer tube group is closest to the combustion means and has a high temperature. ,
Although it is exposed to burning gas, it is cooled by fluid such as water flowing inside, which prevents heat damage. The first group of heat transfer tubes is preferably one without fins on the outer surface because the temperature of the surrounding combustion gas is high and the heat transfer rate is also high due to radiant heat transfer from the radiator described later, but the amount of heat transfer increases. From this point of view, it is also possible to use a so-called loaf-in type having fins with a height of 2 mm or less, for example.

By the way, the present inventors proposed in Japanese Patent Application No. 60-223980 that a plate-shaped air permeable radiator IF! is provided between the first heat exchanger tube group and the second heat exchanger tube group! : We proposed a fluid heating device @ arranged perpendicular to the flow direction of combustion gas. In this structure, the first heat exchanger tube group mainly receives radiant heat from the radiator only on one side on the radiator side, and hardly receives radiant heat on the other side. It is also possible to arrange a radiator along the flow direction of the combustion gas between the heat exchanger tubes constituting the heat exchanger tube group, but in this case, the combustion gas tends to flow along the outer surface of the radiator, and the combustion gas There is a problem that the heat transfer from the radiator to the radiator is not sufficient, and therefore the radiant heat transfer from the radiator to the first heat transfer tube group is also insufficient.

Therefore, in the present invention, a plurality of breathable radiators are arranged between the heat exchanger tubes constituting the first heat exchanger tube group so as to be inclined with respect to the flow direction of the combustion gas.

As a result, the combustion gas passes through the interior of the radiator from one surface to the other surface, and at this time, the thermal energy of the combustion gas is reliably and sufficiently provided to the radiator. The radiator becomes incandescent and emits radiant heat from both sides.

Many of the heat exchanger tubes that make up the first heat exchanger tube group receive radiant heat from multiple radiators located outside them,
The radiant heat receiving area of each heat exchanger tube increases greatly, and radiant heat is effectively transferred from the radiator to this heat exchanger tube. By such radiant heat transfer from the radiator and convective heat transfer from the combustion gas, the first heat transfer tube group is heated, and the fluid flowing therein is heated.

Note that some of the radiant heat from the radiator is also irradiated to the combustion means, but the inclination angle of the radiator may be adjusted or the radiator and P
By arranging a heat transfer tube at the gate to the A-burning means, the amount of radiant heat irradiated to the combustion means can be made relatively small. Therefore, the combustion means will not be burned out by this radiant heat.

Since the radiator is heated to a high temperature, it promotes the oxidation reaction of unburned components such as CO1HC contained in the combustion gas, and also contributes to the purification of the combustion gas.

The second heat exchanger tube group is arranged adjacent to and downstream of the first heat exchanger tube group. That is, it is arranged at a downstream position of the radiator and the first heat exchanger tube group with respect to the flow direction of combustion gas.

This second heat transfer tube group is heated mainly by convective heat transfer from the combustion gas, but also receives radiant heat from the upstream radiator. This heats the fluid flowing inside.

・The temperature of the combustion gas flowing into the distribution area of the second heat exchanger tube group increases due to heat exchange with the fluid in the heat exchanger tubes when passing through the first heat exchanger tube group and the distribution area of the radiator. Therefore, from the viewpoint of improving heat transfer efficiency, it is preferable that the second heat transfer tube group has fins on the outer surface. In the heat tube group, heat transfer tubes can also be arranged in a staggered manner.

According to a preferred embodiment of the invention, the air-permeable radiator is arranged in such a way that its overall longitudinal cross-section is serrated over substantially the entire cross-section of the combustion gas flow path. By arranging the radiator as described above, almost the entire amount of combustion gas is forced to pass through from one surface of the radiator to the other surface, increasing radiant heat transfer from the radiator. Roughly, radiant heat is applied to both the first heat exchanger tube group and the second heat exchanger tube group from multiple directions, and the radiant heat is uniformly transferred to the entire first heat exchanger tube group and the second heat exchanger tube group. This enables efficient combustion heat utilization.

Examples of the breathable radiator include a honeycomb body, a three-dimensional network body, and an open-air foam body in which gas flows through both sides IWl of a plate-like body.

This radiator is preferably made of a heat-resistant material, such as ceramics such as silicon carbide, silicon silicide, cordierite, or heat-resistant steel, so as to generate effective radiant heat at high temperatures.

The heat exchanger tubes of the first and second heat exchanger tube groups may be made of a material with excellent thermal conductivity and corrosion resistance, such as metals such as copper, stainless steel, and aluminum alloys, or ceramics such as silicon carbide and enriched silicon. Preferably, especially
With high thermal conductivity, high emissivity, low coefficient of linear expansion, and high strength,
Most preferred are reactive sintered silicon carbide, which has excellent formability, or copper, which is a highly thermally conductive material.

In the case of the present invention, by placing the first heat exchanger tube close to the combustion means, it is conceivable that the first and second heat exchanger tube groups are subjected to a large heat transfer load and become locally high temperature. Also,
Depending on the conditions, it is conceivable that the fluid to be heated, such as water, may locally boil, and the generated steam may inhibit heat transfer, resulting in a locally extremely high temperature. Therefore, if the heat transfer tube is made of metal with poor heat resistance, it may overheat and oxidize, and in extreme cases, it may melt, so it is desirable to select settings such as material, layout, and usage conditions appropriately. In this respect, if the tube is made of ceramic, sufficient heat resistance can be obtained, and it is particularly preferable for use in a heat exchanger tube in a high temperature section.

Also, t! When trying to recover not only the sensible heat but also the latent heat held by A-burning gas, nitric acid may be generated in the low-temperature heat exchanger (natural gas itself is clean, but the high-temperature combustion (The NOx generated by this combines with moisture condensed on the low-temperature portion of the heat exchanger tube surface to form nitric acid.) From this point of view as well, it is preferable that the low-temperature heat exchange section be made of ceramic, which has corrosion resistance.

For the same reason, the material of the fins provided on the outer surface of the heat exchanger tube in the high temperature section or the low temperature section is preferably ceramic, but metals such as copper and stainless steel can also be used.

"Embodiments of the Invention" Examples of the fluid heating device according to the present invention will be described below with reference to the drawings.

FIG. 1 shows a first embodiment of the present invention. This fluid heating device 111 is entirely surrounded by a casing 12 whose upper part is connected to an exhaust port (not shown).
A fan casing 13, a mixing chamber 14, and a combustion chamber 15 are configured to communicate with each other in order from the bottom. A fan 16 is incorporated in the fan casing 13, and a fuel gas nozzle 17 is disposed at a discharge portion of the fan 16. Air flow from the fan 16 and fuel gas from the fuel gas nozzle 17 are supplied to the mixing chamber 14 via the fan casing 13 to create a mixture of fuel gas and air.

A surface burner plate 18 as a combustion means is arranged at the boundary between the mixing chamber 14 and the combustion chamber 15. The surface burner plate 18 has a large number of flame ports, and the air-fuel mixture that has passed through the flame ports forms a planar flame on the downstream surface of the surface burner plate 18. That is, in this embodiment, a premixed surface burner method is adopted.

Above and close to the surface burner plate 18 inside the casing 12, a plurality of heat exchanger tubes 19a and 19b are arranged horizontally in parallel with each other in a staggered manner to form a first heat exchanger tube group 19. The heat exchanger tubes 19a and +9b have an outer diameter of 12~
20 mm, with a wall thickness of 0.6 to 2.0 mm.

The diameter of the heat transfer tube is small (the ratio of the external surface area of the heat transfer tube per unit volume of fluid flowing inside the window increases, increasing the amount of radiant heat received, but on the other hand, the number of required heat transfer tubes also increases, and the inside The above dimensions are preferable when the present invention is applied to a water heater, since there is a high possibility that the flow path will be blocked by air bubbles when boiling occurs.

It is preferable to arrange the heat exchanger tubes at intervals of 12 to 20 mm, which is approximately the same as the outer diameter of the heat exchanger tubes.If this interval is too small, pressure loss may increase when combustion gas passes, or the radiator may pass through the interval. The amount of radiant heat reaching the rows of heat transfer tubes located further away is greatly reduced.

The distance Ha from the upper surface of the surface burner plate 18 to the underline of the first heat exchanger tube group 19 (including the fins if provided with fins) is within 50 mm. The cross-sectional shape of the heat exchanger tubes 19a and +9b is not limited to the usual cylindrical shape, but may be elliptical.

A breathable radiator 2 is placed between the heat exchanger tubes 19a and +9b.
0 is arranged parallel to the heat exchanger tubes 19a and 19b and inclined with respect to the flow direction of the combustion gas, not only the first heat exchanger tube group 19 but also the second The heat exchanger tube group 21 is also irradiated with radiant heat.

In this embodiment, a honeycomb plate is used for the radiator 20. The radiator tends to have a large temperature distribution between the parts near and far from the heat transfer tube, and the temperature changes greatly when the fluid heating device is ignited and extinguished, so it should be made of ceramic, which has excellent heat resistance and thermal shock resistance. has been done. This honeycomb plate has a large number of parallel cells penetrating the front and back sides of the plate surface, and the cell shape can be appropriately selected from square, rectangular, hexagonal, etc. Further, the honeycomb plate may be formed by laminating a large number of corrugated plates or a large number of corrugated plates and flat plates.

A second heat exchanger tube group 21 is arranged close to and above the first heat exchanger tube group 19 and the radiator 20 . In this embodiment, the second heat exchanger tube group 21 is composed of a large number of parallel flat plate-shaped fins 22 and a plurality of staggered transverse heat exchanger tubes passing orthogonally through the fins 22. For example, each heat transfer tube may have a plurality of fissures formed on its outer surface.

Each heat exchanger tube of the second heat exchanger tube group 21 has an outer diameter of 12 to 20 mm.
, wall thickness 0.6-2.0mm, heat exchanger tube arrangement interval 12-2
0 mm, the wall thickness of the fins 22 is 0.3 to 1.5 mm, and the arrangement interval of the fins 22 is 2.6 to 6.0 mm.

The heat exchanger tubes in both heat exchanger tube groups 19 and 21 are generally arranged horizontally, but in order to make it easier for air bubbles to escape when the heated fluid boils, they are arranged so that the outlet side is higher than the inlet side of the heated fluid. It may be tilted.

A fluid to be heated is flowed into the first heat exchanger tube group 19 and the second heat exchanger tube group 21. A liquid, particularly water, is suitable as the fluid to be heated. This fluid to be heated is supplied to the first heat exchanger tube group 1
9 and the second heat exchanger tube group 21, respectively, but preferably it is flowed in series between them. In this case, in order to increase the temperature efficiency, first the second heat exchanger tube group 2
1, and the fluid to be heated that exits here is then allowed to flow through the first heat transfer tube group 19, so that it flows countercurrently to the flow of combustion gas. On the other hand, local boiling within the tubes is prevented. In order to achieve this, it is preferable to connect the combustion gas in the opposite direction so that the flow is parallel to that of the combustion gas. Furthermore, although it is customary for each heat exchanger tube group to be connected in series,
Series connection and parallel connection may be combined as appropriate.

The operation of the device of the present invention will be explained below.

The fuel gas from the fuel gas nozzle 17 and the air flow from the fan 16 are sent to the mixing chamber 14 to form a premixture.

The premixture passes through the surface burner plate 18 and enters the WS baking chamber 1.
5, it is formed into a flame and becomes combustion gas. The air ratio of the premixture should be set to 1.0 as much as possible in order to maintain a high combustion gas temperature.
However, in order to suppress the generation of unburned components, it is preferable to set the air ratio to about 1.1 to 1.4. As a result,
The combustion gas is generated at a high temperature of 1500 to 1650°C and is guided to the first heat exchanger tube group 19, where a part of the thermal energy of the combustion gas is transferred to the first heat exchanger tube group 19 by convection heat transfer. The heat is transferred to the fluid flowing inside the heat transfer tube.

Most of the combustion gas hits the radiator 20, which is arranged at an angle, while remaining at a high temperature, and flows through the inside of the radiator 20.
Heat enough to make it incandescent. At this time, the radiator 20 is maintained at a high temperature of 800 to 1200°C, and N is placed around it.
The first heat exchanger tube group 19 is heated by radiation from both sides of the radiator 20. The temperature of the radiator should be as high as possible from the perspective of radiant heat transfer, but it should be maintained within the above temperature range, taking into account temperature changes during ignition and shutdown, and temperature differences between the radiant body parts during operation. It is recommended to set it to .

At this time, some of the radiant heat is also irradiated to the surface burner plate 18, but the irradiation is mainly from an oblique direction and the amount of irradiation is much smaller than that to the first heat exchanger tube group 19.
The surface burner plate 18 will not be burned out by this radiant heat.

While the combustion gas passes through the area where the first heat exchanger tube group 19 and the radiator 20 are arranged, its temperature decreases to 600 to 800 °C due to heat exchange with the fluid flowing inside the heat exchanger tubes, and the temperature of the combustion gas decreases to 600 to 800 °C. It is guided to the heat transfer tube group 21 and transfers heat energy again to the fluid flowing inside. At this time, the radiant heat of the radiator 20 is also irradiated onto the second heat exchanger tube group 21 mainly from an oblique direction. In this way, the fluid such as water inside the second heat exchanger tube group 21 is, for example, 4
The hot water becomes hot water at a temperature of 0 to 80°C and is led out of the device.

A performance evaluation experiment was conducted using the fluid heating device 11 of the above example and the fluid heating device of the control example. The experimental conditions and experimental results are as follows.

(Experimental conditions) ■Fuel: natural gas, air ratio 1.2 ■Heated fluid for two people Water at a water temperature of 20°C is first flowed through the first heat transfer tube group 19, and after leaving here, it is passed through the second heat transfer tube group 19. It flows into the heat tube group 21.

■First heat exchanger tube group 19: inner diameter 11.4 mm, outer diameter 12.
7mm.

Five copper loaf-in tubes with height I of fin +9c of 6 mm and outer diameter of fin 19c of 15.9 mrn are arranged in a staggered manner at an interval of 16 mm (pitch approximately 32 mm).

■Radiator 20: plate thickness 5mm, number of cells 200/in2
, pressureless sintered silicon carbide honeycomb plate 2 with square cell cross section
Arrange the sheets at an angle as shown in Figure 1.

■Second heat transfer tube group 21: Copper tubes with inner diameter of heat transfer tubes of 11.5 mm and outer diameter of 12.7 mm, and fin thickness of 0.35 mm.
, a plate fin tube that combines fins with a fin pitch of 2.7mm.

In addition, in the apparatus of the embodiment, the distance jf from the upper surface of the surface burner brake H8 to the lower assembly of the fins 19c of the heat exchanger tube +9b
la 820 mm, and the distance mb from the top surface of the surface burner plate 18 to the lower edge of the fins of the second heat exchanger tube group 21 is 68 mm.
And so.

In addition, in the device of the control example, the first heat exchanger tube group 19 and the radiator 20 were not provided, and only the second heat exchanger tube group 21, which was the same as that used in the device of the example, was provided, and the distance 111b 18
:200mm.

(Experimental results) Combustion air circumferential volume + heat exchange section volume Control example: Example = I: 0.28 Heat transfer area Control example: Example = I: 1.18 Exchange heat amount Control example: Example = I: 1. 64 As can be seen from this result, in the apparatus of the example, the first heat exchanger tube group 19 and the radiator 20 were arranged in the above relationship in the tri-firing chamber 15, so the combustion space was significantly reduced. In addition, by increasing the heat transfer area by 18%, the amount of heat exchanged was increased by 64%. Also, emission C
The O concentration was also decreased in the example compared to the control example. In addition, in the apparatus of the example, the combustion chamber load was increased to about 30 times that of the control example.

FIG. 2 shows a second embodiment of the invention. This embodiment differs from the first embodiment in that the plurality of breathable radiators 20 are arranged at an angle so as to cover the upper heat exchanger tubes 19a of the staggered first heat exchanger tube group 19 from both sides. The longitudinal section of the radiator 20 has a sawtooth shape as a whole. Therefore, the radiator 20 is disposed in the combustion gas flow path over substantially the entire area across the casing 12, and the entire amount of combustion gas passes through the radiator 20. Therefore, the thermal energy of the combustion gas is sufficiently transferred to the radiator. Furthermore, the space required for arranging the radiator 20 is also reduced. Moreover, since the direction of irradiation of the radiant heat by the radiator 20 is bidirectional, the radiant heat is uniformly irradiated to the entire first heat exchanger tube group 19 and the second heat exchanger tube group 21, and combustion heat can be efficiently recovered. can. Other configurations and operations are similar to those in the first embodiment, so their explanations will be omitted.

"Effects of the Invention" As explained above, according to the present invention, convective heat transfer by the combustion gas and radiant heat transfer by the radiator act simultaneously on the first and second heat exchanger tube groups. Combustion heat can be used effectively, and the heat transfer effect can be improved. Moreover, since a considerable amount of the radiant heat from the radiator is blocked by the first heat transfer tube group, burning out of the combustion means due to the radiant heat is prevented. In general, since the radiator is kept at a high temperature, even if incomplete combustion products occur in the combustion chamber, the oxidation reaction is promoted by this high temperature radiator, reducing the amount of incomplete combustion products emitted. Can be suppressed.

Furthermore, the overall size of snow making can be reduced.

[BRIEF DESCRIPTION OF THE DRAWINGS] Fig. 1 and Fig. 2 show the first and second parts of the fluid heating device of the present invention, respectively.
FIG. 2 is a cross-sectional view of an embodiment and a second embodiment. 11 is a fluid heating device, 12 is a casing, 13 is a fan casing, 14 is a mixing chamber, 15 is a combustion chamber, 18 is a surface burner plate, 19 is a first heat exchanger tube group, 20 is a radiator,
21 is a second heat exchanger tube group, and 22 is a fin. 'i41 Figure 2Fs

Claims (1)

[Claims] 1. Between the combustion means, the first heat exchanger tube group disposed adjacent to and downstream of the combustion means, and the heat exchanger tubes constituting the first heat exchanger tube group, in the flow direction of the combustion gas, 1. A fluid heating device comprising: a plurality of air-permeable radiators disposed at an angle; and a second heat exchanger tube group disposed adjacently downstream of the first heat exchanger tube group. 2. The fluid heating device according to claim 1, wherein the permeable radiator is arranged so that its longitudinal section as a whole has a sawtooth shape over substantially the entire cross section of the combustion gas flow path.