FOR

COOKING SYSTEM

FIELD OF THE INVENTION

The invention relates generally to cooking equipment. In particular, the invention relates to heat transfer efficiency from a flame source from combustion of natural gas to a wok in a wok range.

BACKGROUND

Gas is a major energy source for cooking. Cooking on a wok has been a major cooking method for Asian cuisines for centuries. One characteristic of wok cooking is that it requires high power burners to quickly heat up the wok to high temperatures. High temperature wok cooking tends to preserve nutrients, and this style of cooking is becoming more popular in family style restaurants and hotels as the 'healthy eating' trend continues.

A typical gas range has a massive array of ring burners or jet burners that shoot out flame towards the wok. The power rating of the burners ranges from 60kBtu/hr to 200kBtu/hr. The wok range is notoriously inefficient, with an energy efficiency of about 15% according to reports from the PG&E Foodservice Technology Center.

There are several reasons for this inefficiency: the fast-flowing jets tend to have incomplete combustion; high jet speeds cause turbulence that can promote intermixing with cold air, especially in larger wok ranges; the wok range body loses heat in the form of infrared radiation. It is typically found that the wok range body will become so hot that it needs to be cooled by running water. A wok range also tends to have a large burner chamber, where there is ample surface area around the chamber to allow radiative heat loss. Due to such a high power, low efficiency setup, the status quo is: lots of gas is wasted, lots of water is used to cool down the range; the chamber wall material deteriorates quickly due to high temperatures; enormous amounts of C0 ₂ are unnecessarily emitted. There is an inherent need to cost-effectively improve efficiency to allow wide application of energy efficient wok ranges to achieve energy savings on a large scale.

To improve the energy efficiency of cookware on gas ranges, we invented a new type of cookware called the Turbo Pot (patent US8037602), which has a finned heat sink to increase the surface area to improve the heat transfer. The new cookware is capable of improving efficiency significantly— it can cut the time required to boil water by 30-50%. More detailed results can be found in the report from the PG&E Foodservice Technology Center.

A natural extension of this heat sink technology was to the wok. The fin structure is a set of concentric rings conforming to the spherical structure of the wok. We have some perforations in the rings to encourage swirling movement of hot air once it enters the structure. The prototype wok was tested by the PG&E Foodservice Technology Center. The heat up time was shown to be reduced by 31% and the energy efficiency increased from 15% to 19% in this preliminary attempt at wok efficiency improvements. Even with the advanced heat sink technology applied on the cookware, the energy efficiency is still less than 50%. Continued improvements on fin structure may not improve much, as it is limited by the choice of the material and fin attachment method. Improving the heat transfer from the cookware side can only take us so far. Therefore, further improvement should come from overall cooking system design.

SUMMARY OF THE INVENTION

To further address the several factors highlighted above that affect the energy efficiency of the gas powered wok burner, we propose here to provide a solution to improve the heat transfer efficiency by managing the flame flow in the wok range.

It is also an objective of current invention to provide a means to reduce heat loss in the burner chamber, and reduce the infrared heat lost in a wok range.

It is also an objective of current invention to provide means to increase the heat transfer path of the flame with the cookware for better heat transfer. It is also an objective to increase the flame to cookware interaction to improve heat transfer. It is an objective to provide an optimized overall cooking system design consisting of specialized cookware and gas burners to improve efficiency.

BRIEF DESCRIPTION OF THE FIGURES

Objectives and advantages disclosed herein will be understood by reading the following detailed description in conjunction with the drawing, in which:

FIG. 1 shows a conventional wok range.

FIG. 2 shows a wok range with a flame concentrator.

FIG. 3 shows a wok range with a flame management device.

FIG. 4 shows a flame perturbation pattern.

FIG. 5 shows a wok range with a power burner.

FIG. 6 shows a wok range with an extended flame path.

FIG. 7 shows a cooking system with extended flame paths.

FIG. 8 shows a cooking system with swirl inducing inward pointing jets.

FIG. 9 shows a cooking system with inward pointing jets and swirl inducing fins.

DETAILED DESCRIPTION

Although the following detailed description contains many specifics for the purpose of illustration, anyone of ordinary skill in the art will readily appreciate that many variations and alterations may be made.

Cooking on a wok is a major cooking method in Asian cuisines. A typical setup is shown in FIG 1, where a wok 101 sits on a wok range 110. A wok is a special kind of cookware where the base of the cookware has a curvature good for stir frying. Due to the concave curvature, the wok base is functioning as the wall of the cookware as well to hold contents. A wok is typically made from steel or stainless steel. Same is true for wok ranges. A gas burner 120 is typically an array of ring burners, jet burners, or duck bill burners. While the burner is on, the flame will rise due to buoyancy. There will be turbulence in the flame and intermixing of the flame with colder air around the column of the flame flow. There is infrared radiation from the flame column heating the wall 110. The flame chamber 130, where the flame fully combusts, has a large surface area to absorb infrared radiation. The large surface area of the flame chamber allows it to transfer heat to the framework of the wok range. The body of the wok range is typically so hot that running water is required to cool it down.

The flame from the burner comes up pretty uniformly reaching the bottom of the wok and intermixes with the air inside the chamber. As the flame is in contact with the bottom of the wok, it transfers heat to the wok while moving along the bottom of the wok before escaping from underneath the edge of the wok. It is expected that the longer the path a flame particle travels along the wok body, the more effective the heat transfer can be.

The first embodiment of the proposed here in this invention is shown in FIG 2, where the wok 201 is placed on top of wok range 210. A burner 220 is placed inside the wok range. A cone shaped flame conduit 230 is placed inside the wok range. This device is made of steel of a certain thickness. The metal is formed in a cone shape with a larger opening 232 placed over the burner to couple the flame into the conduit. The flame will exit the narrower exit opening 233 to reach the wok. The narrower opening of the conduit on the top will enable a longer heat exchange path for the flame to transfer heat to the wok, compared to a flame of identical power with uniform intensity over the whole wok surface.

To demonstrate this mathematically, one can assume that the heat transferred by a flame particle is an increasing function of the distance it travels along the surface of the wok, or the distance from the initial point of contact to the edge of the wok. We further assume that the heat transferred approaches a finite limit exponentially as the distance the flame particle travels approaches infinity (heat left in the particle decreases by a factor of e every three inches it travels), so this can be written as dQ <x ( 1— e 3 J dF where dQ is the transferred heat, r is the distance from the center that the flame particle hits the wok, R _w is the radius of the wok and dF is the amount of flame.

Let f(x) be the probability distribution function of flame particles as a function of distance from the center. If we assume that flame particles are uniformly distributed in a circle of radius R _f which is defined by exit opening 233 of the flame conduit, f(x) =— ⁼ ~-

Then dF = f(r)dr , so dQ = ( 1— e 3 ) f(r)dr and therefore Q °c / _{r= 0} ( l e If Q _Q represents the heat transferred to a wok with

7inch radius from a flame of 6 inch radius, and Q ₁ represents the heat transferred with a wok of 7 inch radius with 4 inch flame radius, it is found that— = 1.16, an increase of

Qo

15% in heat transferred.

The opening 232 is large enough for good combustion of natural gas, so there is no need for further intake of air for further combustion. This is especially true for ring burners. The wall of the device reduces the intermixing of colder air with the flame along the upward path of the flame.

The outer wall of the conduit can also be coated with layers of thermally insulating materials, such as high temperature fiberglass or high temperature ceramic. Alternatively, the device can be formed directly using such high temperature thermally insulating materials. The outside surface of the device is coated with materials with low emissivity. With this thermally insulating, low radiation arrangement, heat loss due to IR radiation will be reduced.

The second embodiment of the current invention is shown in FIG. 3. A wok 301 is placed on the wok range 310 with burner 320. There is a flame management device 330 consisting of cone shaped flame conduit 331 as the lower portion and annular flange 332 as the upper portion. The inner perimeter of the annular flange joins the smaller opening of the cone. The functions of the cone 331 are to concentrate the flame to the center of the wok base; reduce the intermixing of hot flame with cold air; and reduce IR radiation. The speed of the flame also increases due to the cone. As the flame comes up to impinge on the base of the wok 300, it tends to bounce off the wok depending on the speed of the flame. This will weaken the interaction of the flame with the wok after it hits the wok. The flange portion 332 of the device is used to confine the flame flow close to the surface of the wok. This will promote interaction of the flame with the base of the wok. Preferably, the shape of this flange will conform to the shape of the wok base. This portion can be constructed with steel coated with thermally insulating materials, or directly formed using thermally insulating materials. The outside surface can be coated with a low IR emissivity coating. The distance between the wok 301 and flange 332 can be adjusted to optimize the interaction and flame flow speed for best heat transfer.

To further promote the interaction of the flame with the base of the wok, it is suggested to create some protruding features on the surface of the flange facing the wok to perturb the flame flow to create turbulence. As shown in the figure, perturbation fins 333 are built on the top surface of the flange 332. The feature pattern on the top flange can also be designed to encourage the flow to swirl about the central axis of the wok range. This will increase the length of the flame path on the wok surface. FIG 4 shows an example pattern of the perturbation features that facilitate the swirling movement in the flame flow. If the features are made with metal, some portion of the features may touch the wok to increase heat conduction as well as support the wok. Preferably, these perturbation features will be connected to the metal flange with small connections to limit heat transfer to the plate.

As heat is concentrated to a small central spot, there will be a heat spot at the center of the cookware base. This is acceptable for boiling water for pasta, etc. However the hot spot can be alleviated by placing a metal plate with holes at the center beneath the cookware, or by changing the flame conduit to one with a larger diameter. The size and number of the holes will vary for certain applications. Tradeoffs will be made between energy efficiency and the required heat uniformity for these applications.

Alternatively, instead of using a cone for concentration, it is possible to just use a powerful burner with a concentrated flame. FIG 5 shows a schematic of such a wok range. Wok range 500 shows a wok 501 on top of a wok range 510. There is an insulation block 530 which has a top surface 531 conforming to the shape of the wok. The block is placed close to the wok to reduce radiative heat loss. The top surface of the block if fit with a polished metal plate. The polished surface of the metal plate will reflect the downward radiation back up toward the wok. Or the surface of this metal plate is coated with coating with high reflectivity in composition infra-red (IR) radiation wavelength ranges of fuels such as natural gas, propane gas, and butane gas. The heat loss downward through the block will be significantly smaller than that through the chamber walls in FIG 1. A power burner 520 is placed inside and under the block 530 and the output of the burner 521 is placed inside the passage 532 of the block 530. The gas line 522 provides fuel to the burner. An atmospheric burner will have some openings 523 to provide air supply like the design of a Meker burner, or a Teclu burner. Alternatively, the burner can have a blower 525 to ensure a good air/fuel mixture to have complete combustion and increase the power of the burner. As the flame shoots out from the burner, it will impinge on the center area of the wok bottom. The impingement provides good heat transfer as the thermal boundary layer in the area is thin. After the impingement the flame flow will be guided through the space between the wok and the flange. The gap between wok and the flange 503 can be set from 10mm to 60mm. The materials used for the insulation block 530 can be fiberglass type thermal insulators. To further improve the flame interaction with the wok, a perturbation fin structure 532 can be built on the flange 531. The height of the fins may increase with distance from the center. The function of the structure is threefold: to confine the flame to flow close to the wok, create turbulence in the flow to reduce the thermal boundary layer, absorb heat from flame flow far away from the wok, i.e. beyond the boundary layer thickness, and radiate or conduct said heat to the wok. These perturbation fins on the flange can also be in other patterns such as the spiral pattern in FIG 4 to extend the length of heat transfer interaction. Perturbation fins can touch the wok for direct heat conduction and guide the flame flows in the spiral curves. It is also possible to have perforations on the fins to allow flame to pass through and create turbulence. Preferably the perturbation fins are connected to the metal flange with small footings so that the heat transfer from the fins to the flange is limited. Also it is possible to make the perturbation fins to be flexible to have good contact with the wok when the wok is placed on top of them.

In this configuration, the flame impinging on the center of the wok will travel the path length of approximately the radius of the wok before it escapes. For an increase in the path length, it may be advantageous to have the impingement spot off-center and direct the flow towards the center of the wok. This will result in a longer interaction path than the radius of the wok. FIG 6 illustrates the principle of this arrangement. Wok 601 is placed over the wok range 610. The wok range 600 consists of a metal plate 631 which is made of steel and has a contour matching that of the wok. The diameter of the plate is about 2/3 of the diameter of the wok. The plate is placed under the wok, at a distance about 10 to 60mm. Under the metal plate there is a block 630 made of high temperature thermal insulator. There is an air gap 632 between the metal plate 631 and the thermal insulation block 630. The distance between the metal plate and block is about 5- 10mm. The function of this air gap and the block is to reduce the heat transfer downward. Air is good insulator, vacuumed is even better. So the metal plate can be a composite structure of two metal plates with vacuum sealed around the edge of the plates. The top surface of the plates will be polished to reflect IR radiation upward. Better yet, it can be a multiple plate structure. There is an array of burner jet ports 620 at one side of the metal plate. The jet ports are pointing toward the center of the wok horizontally, and are tilted upwards slightly so the flame will impinge on the wall of the wok. The placement of the jet ports is such that the impingement location on the wok is at about 2/5 of the way from the center to the rim of the wok. After the impingement, the majority of the flame continues to flow towards the center of the wok along path 640 and exits at the other side of the rim. The majority of flame travels across the cooking area of the wok for a length of 1.4 times of the radius, compared to the conventional center firing configuration where flame travels the radius of the cooking area at most; therefore, the efficiency is higher. If the effective cooking area is about 2/3 of the radius, where we only consider the flame flow by the effective cooking area, the improvement of the path length is 1.6 times. The tradeoff is that heating uniformity will not be as even as that of a conventional flame pattern. However, since stir frying is a typical cooking method for wok cooking, heating uniformity is not a major concern.

It is also desirable to have the flame flow to be close to the wok wall after the impingement, therefore a perturbation fin structure 633 is built on the metal plate 631. The perturbation fins are raised up from the plate and force the flame to flow close to the wok surface. At the same time the fins create some turbulence in the flow to improve heat transfer. Some perforations in the fin may help. Also the fins reflect some of the flow towards the areas 641 and 642 away from the major flame flow path 640 to provide more heating uniformity. Alternatively some portion of the perturbation fins can be touching the base of the pot to further improve the heat transfer.

Such a burner configuration is suitable for use with linear finned energy efficient cookware as shown in US patent no. 8037602. When the burner is placed on one end of the heat exchange channel formed by the fins on the cookware, the flame can travel though the whole length of the exchange channels.

In an effort to make heating more uniform, it is possible to arrange the impingements to be more centrally symmetric. One attempt to do so is shown in FIG.7, depicting an improved gas range top burner whose principle can be applied to wok ranges as well. In FIG.7 the gas range burner consists of a round metal plate 731 placed above a thermally insulating block 730. The diameter of the plate is about the diameter of the cookware to be used. A cookware not shown in the figure typically has a base and a wall. There is a space between the metal plate and the insulating block of about 5- 10mm. There is an array of burner jet ports 720 installed at the perimeter of the plate 710. Circular fuel pipe 721 delivers gas to the jet ports. The burner jet ports point toward the center of the burner, and are tilted upward so that the flame coming out of the ports will impinge on the base of the cookware. The impingement point is at about 1/5 of the distance from edge to the center of the cookware base. There is an arrangement of metal fins 733 dividing the jet ports into groups. Four groups as shown in FIG.7 while larger number of groups increases heating uniformity. Within the space around each jet port group, there are fins 734 to separate the space into the inflow path 751 , where the jet ports are situated, and the outflow path 752, where there are no jet ports. Flame from the burner jet ports will impinge on the cookware base and flow toward the center of the burner guided by the fins along the inflow path. Near the center of the burner, the fin 733 is bent such that it will guide the flame flow to swirl around the center. There is a cone shaped protrusion 735 at the center of the plate 710 to force the flow to interact with the base of the cookware. While swirling around the center area, some of the flame will be encouraged by fin 733 to exit via outflow path 752. So the flame flow will flow through the inflow and outflow paths, having a longer path than a conventional central firing burner for better heat exchange. The guide fins as shown are straight fins, but it is preferable to have fins following a curved shape, such as the spiral shape shown in FIG.4 to force a swirling motion in the flame flow.

To further improve the heat transfer along the path, some perturbation fins 760 will be installed in the inflow and the outflow paths. The fins 760 will promote interaction of the flame with the cookware and at the same time regulate the speed of the flow. The size of fins and the dimension of the inflow and outflow paths will be optimized so that sufficient air is provided for good combustion. The flame speed is adjusted to create pressure in the center region to have good thermal transfer to the cookware, and smooth flame flow to the outflow paths without backflow of flame to the inflow path. Fins 733 and 734 may have some perforations or undulations along their lengths to enhance turbulence and heat transfer.

To increase power, multiple arrays of the jet ports can be added to the structure. To increase uniformity, the fins divide the jet ports to a larger number of groups. To further reduce the radiative loss, the metal plate 731 can be a double plate structure which is vacuum sealed at the edge of the plates. There is vacuum in between the plates resulting only limited thermal conduction via the edge of the plates. The top surface of the plates is coated with coatings having high reflectivity in IR wavelength range from combustion of fuels such as natural gas, propane gas, and butane gas. This metal assembly will be mounted on chassis of the range via small metal connectors or via ceramic or other thermally insulating fixtures.

It is also beneficial to have fins built on the base of the cookware like shown in US patent no. 8037602. Preferably, the fins on the cookware are in a radial pattern to form fin channels. The flame from inward flame path will travel in the cookware fin channels toward the center of the cookware. There will be an open area in the central area of the pot that is free of cookware fins to allow the flame from cookware fin channels in the inward path to transfer to the cookware fin channels in the outward path. The combination of extended surface area of the cookware and the extended flame path will provide a substantial improvement of fuel efficiency over conventional cookware on a conventional stove top.

The burner jet ports can be deployed along the entire perimeter of the burner to further improve the uniformity. This design is shown in FIG 8. A cookware 801 is placed on top of burner 800. Cookware 801 has a base and a wall. The burner 800 has round metal plate 831 which diameter is similar to that of the cookware base. The metal plate is placed on top of an insulation block 830. There is a gap 832 between the metal plate and the insulation block. The gap is about 25mm; the larger gap is for exhaust flow to pass through. There is a hole 833 in the center of the metal plate. The metal plate can be bent upward towards the center hole. Burner jet ports 820 are uniformly deployed around the metal plate. The jet ports point toward the center of the burner and are tilted upward so that the flame will impinge upon the bottom of the cookware. The area of the impingement will be a circle about 4/5 of the diameter of the cookware, concentric with the cookware. The jet ports do not point directly toward the cookware 's central axis; rather, they are horizontally rotated 45 degrees with respect to that axis. Because of this angle, the jet flame from each port will move towards the center of the burner and, at the same time, move in a circular direction. As the flame swirls around the cookware, it cools down and contracts towards the center. The cooler portion of the flame flow will be forced down hole 833 by buoyance and come out through the gap 832 between the plate 831 and the insulation block 830. The size of the hole 833 should be small enough to allow the flame to build up in the center area so that it will interact with the cookware efficiently, while large enough to allow the exhaust to flow through the gap without affecting the inward flow of hotter flame.

Again it is beneficial to have fins built on the base of the cookware like shown in US patent no. 8037602. For this burner, the fins on the cookware are preferable to be in radial pattern to form fin channels. The direction of said channels in the radial pattern will match the direction of the jet ports so that the flame flow from the jet ports will enter and travel in the cookware fin channels. The combination of extended surface area of the cookware and the extended flame path will provide a substantial improvement of fuel efficiency over a conventional cookware on a conventional stove top.

An alternative arrangement is shown in FIG 9, where burner 900 has round metal plate 931. The metal plate is placed on top of a thermally insulation block 930. There is a gap 932 between the metal plate and the insulation block. The gap is about 25mm; the larger gap is for exhaust flow to pass. There is a hole 933 in the center of the metal plate. The metal plate is bent upwards toward the center hole. An array of burner jet ports 932 are uniformly deployed around the metal plate. The jet ports point towards the center of the burner and are tilted upwards so that the flame will impinge upon the bottom of a cookware. The area of the impingement will be a circle about 4/5 of the diameter of the cookware, concentric with the cookware. There are some perturbation fin 934 built on plate 931. The fins are tilted in such way to cause the flame flow to swirl under the cookware. As the flame swirls around the cookware, it cools down and contracts toward the center. The cooler portion of the flame flow can be forced down hole 933 and come out through gap 932 between the plate 931 and the insulation block 930. The size of hole 933 should be small enough to allow the flame to build up in the center area so that it will interact with the cookware efficiently, while large enough to allow the exhaust to flow through the gap without affecting the inward flow of hotter flame. Alternatively the fins structure can be the in pattern shown in FIG.4 to induce swirl movement in the flame flow. Again it is beneficial to have fins built on the base of the cookware like shown in US patent no. 8037602. For this burner, the fins on the cookware are preferable to be in radial pattern to form fin channels. The direction of said channels in the radial pattern will match the direction of the jet ports so that the flame flow from the jet ports will enter and travel in the cookware fin channels. The combination of extended surface area of the cookware and the extended flame path will provide a substantial improvement of fuel efficiency over a conventional cookware on a conventional stove top.

It will be valued to those skilled in the art that the preceding examples are exemplary and not limiting. It is intended that all permutations, enhancements, equivalents, and improvements thereto, that are apparent to those skilled in the art, upon a reading of the specification and a study of the drawings are included within the true spirit and scope of the present disclosure. It is therefore intended that the following appended claims include all such modifications, permutations and equivalents that fall within the true spirit and scope of the present disclosure.