NOZZLE TEMPERATURE CONTROL

The present invention relates to the field of controlling the temperature of a nozzle for formation and administrating of droplets of a first fluid into a stream of a second fluid. The present invention relates in particular to a device and a method for controlling the temperature of a nozzle administrating urea in form of droplets into an exhaust system.

INTRODUCTION TO THE INVENTION

It has been found that introduction of urea into the exhaust gasses streaming from a combustion engine and into a catalytic system may dramatically increase the efficiency of the catalytic element's capability to convert NOx gasses. While urea in it self is relatively harmless to the environment and the amounts introduced into the combustion system thereby can be overdosed, such wasting of urea is often undesirably as the technology is often applied to moving vehicles and such waste would require larger storage capacities than what is actually needed if urea is dosed correctly.

A need for introducing the required amount of urea into the exhaust gasses only is therefore present. Furthermore, urea is most efficiently introduced into the exhaust gasses as a spray of droplets which typically requires that the urea is pressurised and fed to a nozzle.

Administrating urea into an exhaust system has shown to be a delicate matter. Urea in the present content is often liquefied by dissolving it in water which liquefied urea is made into droplets fed to the exhaust system by a nozzle. As urea in pure form is a salt and liquefied urea containing water giving rise to a number of reactants (often referred to as urea derivatives) under certain temperature conditions, the temperature range in which the nozzle is going to operate is a highly delicate matter.

Typically, the temperature at the location where the nozzle is located in an exhaust system is in the order of -45 to 65O ⁰ C. This interval covers start up of an engine at extreme cold locations and operating at extreme hot locations. In particular, during start-up of the engine, the temperature at the location where the nozzle is located is comparable with the temperatures of the surroundings, that is typically in the range of -45° to +85 ⁰ C. This may imply that the nozzle on one hand may experience changes in geometry resulting in e.g. leakages and on the other hand be so cold that the liquefied urea freeze. In addition, an elevated temperature of the nozzle may be used to ensure that urea derivatives coming from the liquefied urea can be removed from the nozzle by the stream of liquefied urea through the nozzle. In contrast, a low temperature may prevent removal of urea derivatives from the nozzle.

Furthermore, if the temperature of the nozzle, when containing urea, reaches a high level e.g. 500 ⁰ C or more, the nozzle may be blocked by crystallized urea derivatives which must be removed manually in a process requiring dismantling the nozzle and servicing the various parts in a part-by-part manner.

It has been found in connection with the present invention that an optimal operation point of the nozzle is 150 to 200 ⁰ C

European Patent Application no. EP 1 672 191 discloses a reducing agent supply unit using a detection signal of the exhaust gas temperature from a temperature detection device to set a supply quantity at or above a lower limit for cooling the interior of an injection nozzle to below the temperature at which urea water crystallizes for the detected exhaust gas temperature and supplied urea water to the injection nozzle at the set supply quantity. Thus, the amount of delivered urea water to the exhaust system is determined by the cooling need.

BRIEF DESCRIPTION OF THE INVENTION

It is an object of the present invention to mitigate blockage of the nozzle by crystallizing urea or urea derivatives due to operating the nozzle at temperatures which could cause blockage of the nozzle.

Thus, in a first aspect the present invention relates to a nozzle for atomization of one or more fluid, said nozzle is adapted to atomize a liquid being fed to the nozzle so as to form droplets of the fluid, - to be arranged and operate to spray droplets into a stream of fluid with temperatures changing at least 150 ⁰ C by comprising means for at least partly insulating the nozzle from the stream of fluid, means for cooling the nozzle and/or means for heating the nozzle.

In the present context a number of terms are used and should normally be construed in accordance with the ordinary meaning derived by a person skilled in the art. However, the meaning of some of these terms is indicated below so as to make the description of the invention more readable.

The temperature change of 15O ⁰ C of the fluid at which the nozzle preferably should be adapted to be arranged and operate to spray droplets is particularly relevant when the fluid is exhaust gas from a combustion engine. In such cases the temperature of the exhaust gasses varies as a function of operation time of the engine and the effect produced by the engine and the present invention aims preferably at nozzles and methods being able to spray droplets into exhaust gasses in most and preferably all running conditions of the engine.

Although the liquid being fed to the nozzle may have a cooling or heating effect on the nozzle depending the temperature difference between the liquid and the nozzle, the insulating means, the cooling means and/or the heating means is/are preferably chosen so that the amount of liquid delivered by the nozzle is controlled with reference to a demand - preferably including a demand for zero amount - for an amount of atomised liquid and the insulation, cooling and/or heating being applied to such an extend that this demand can be fulfilled.

The term liquid is used to comprise at least a liquid or liquefied reducing agent, such as liquefied urea and/or a liquid/liquefied substance acting chemically like urea preferably to reduce NO _x when introduced into an exhaust system.

Droplets is to be understood in broad sense and includes preferably drops, mist etc.

The terms "liquefied urea" are in general used to designate a mixture of urea salt and water.

An exhaust system as considered herein preferably comprising a piping system connected to a combustion engine. The exhaust system preferably comprising a catalytic converter and the nozzle according to the present invention is typically arranged upstream of the catalytic converter. The terms "active cooling" are in general used to designate a design in which the nozzle is kept cool typically below 100 ⁰ C at all time. This is preferably done by an active element, e.g. an element driven by electricity or other mean of power supplied, e.g. cooling compressor, peltier element, liquid nitrogen etc. This is preferably done in order to make sure that e.g. liquefied urea does not changes state and makes blockages The terms "passive cooling" are preferably used to designate a design in which no, or substantially no external power, is supplied to the nozzle for transmitting heat away from the nozzle. Typically, passive cooling design may comprise cooling fins, heat insulation or combinations thereof.

The terms "selective cooling" are preferably used to designate a system in which no, or substantially no external power, is supplied to the nozzle for transmitting heat away from the nozzle and wherein a substantial heat transport sets in when the temperature of the nozzle reaches a selected level. Typically, a selective cooling design may comprise a liquid evaporation-condensing device where the selected level is set by the evaporation temperature of the liquid. In addition to cooling, it may in certain circumstances be useful to heat the nozzle; this is typically used for melting deposits which block e.g. flow passages on the nozzle. In this context, heating is preferably referred to as "active heating" which terms are preferably used to designate a design in which the nozzle during normal operation is kept at not to high temperature by e.g. convection cooling to the surrounding and shielding towards the exhaust stream. At start up or blockage the nozzle may be heated by an electric heat element that may ensure that the restart temperature is achieved.

By atomization is preferably meant that one or more fluid streams are decomposed into smaller units, such as droplets. This process is often referred to as droplet formation.

In another aspect, the present invention relates to a method for controlling the temperature of a nozzle, said nozzle comprising one or more of features according to any of the preceding claims, the method comprising one or more of the following steps: activate or de-activate the heating means activate or de-activate the cooling means to respectively raise or lower the temperature of the nozzle. The invention and in particular preferred embodiments thereof will be disclosed in connection with the accompanying figures in which:

Fig. Ia and Ib show a first embodiment of nozzle cooling design employing passive cooling according to the present invention; Fig. Ia shows a longitudinal cross sectional view of the overall nozzle design and fig. Ib shows details of a nozzle block

Fig. 2 shows in a longitudinal cross sectional view a second embodiment of a nozzle cooling design employing passive cooling according to the present invention,

Fig. 3 shows in a longitudinal cross sectional view a third embodiment of a nozzle cooling design employing active heating according to the present invention, Fig. 4 shows in a longitudinal cross sectional view a fourth embodiment of a nozzle cooling design employing selective cooling according to the present invention,

Fig. 5 shows schematically a preferred embodiment of a nozzle according to the present invention,

Fig. 6 shows schematically an arrangement of a nozzle according to the present invention, and Fig. 7 shows schematically another arrangement of a nozzle according to the present invention. DETAILED DESCRIPTION OF THE INVENTION

In the following different embodiments of nozzle cooling designs will be disclosed in details. Although other cooling techniques and in particular combinations thereof are available which are clearly within the scope of the present invention, the embodiments disclosed below focus on "passive cooling", "active cooling" and "selective cooling" of the nozzle.

Especially, the active cooling, and active heating concepts may require that the temperature of the nozzle is sensed and the signal provided by this sensing directed to a controlling unit which control the cooling and/or heating. In designs embodying such concepts, a temperature sensor is typically built-in the nozzle typically close to a position where fluid leaves nozzle. The signal generated by the temperature sensor is directed to a controlling unit which comprises a CPU and a code being adapted to convert the signal from the sensor to a control signal to the active, selective cooling or heating means so as to active or de-activate these means if the temperature sensed is at a pre-selected level.

Such active means is typically used to prevent blockage of the nozzle. For instance, if the nozzle is so cold that blockage may occur or has occurred, active heating may remove the blockage of the nozzle. In another scenario, the nozzle would during operation be so hot that urea derivative tends to or are blocking the nozzle and in this case active cooling, selective and/or passive cooling may prevent urea derivates in blocking the nozzle.

In fig. 1 a first embodiment of a nozzle according to the present invention is disclosed schematically. The nozzle comprising the following elements: 1 : nozzle block dry side; 2: nozzle block wet side; 3: membrane; 6: connection tube; 7: outer tube; 8: bottom brick; 9: heat conductive block; 11 : connection for tube; 12: silicone rubber; 13: ceramic insulation; 14 outer bushing; 15: insulation; 16 safety screen; 17: upper clamp; 18: lower clamp; 19 screw; 20 locking wire; 21 : spring ring. The nozzle is arranged in the wall 35 of an exhaust pipe by a flange 51. Lower part of fig. 1 shows inter alia details of the nozzle block and the outer tube 7 as seen from an upstream direction with reference to the flow direction of the liquefied urea.

With reference to fig. 1, showing the first preferred embodiment of a nozzle cooling design, atomization is done by the nozzle block comprising elements 1, 2, 3 which sprays through the opening of element 8 and 13. Element 8 is support for the nozzle blocks in order not to stress the ceramic insulation 13. The ceramic insulation 13 works as a heat shield for the nozzle block elements 1, 2, 3.

Atomization is performed by the nozzle block elements. Fig. Ib is a perspective view of an embodiment of a nozzle block 1, 2, 3 wherein a membrane 3 is provided between the nozzle block dry side 1 and the nozzle block wet side 2. The membrane 3 is provided with channels 3a for guiding the fluid flow. The channels 3a extend partly or wholly (which is shown) through the membrane 3 and are in fluid communication with the fluid outlet 2a of the nozzle block wet side 2. The channels 3a are open and their converging openings terminate in a side of the channel spacer 3. The other surfaces of the elements 1, 2, 3 are shown substantially planar.

Penetrations 3b in membrane 3 are used to orientate the membrane relatively to the block 1 and 2. In a particular embodiment, block 1 comprises elevations (not shown) mating the penetrations 3b and block 2 comprises holes which also mates the elevations.

Liquefied urea is fed to the nozzle 0 at the connection 11 which is fluid communication with the connection tube 6. The tube 6 is connected to the inlet 2b in the nozzle block wet 2 side which is in fluid communication with the outlet 2a of nozzle block 2. Thus, when liquefied urea is fed to the nozzle liquefied urea flows to the nozzle blocks and out through the channels 3a. As these channels 3a are converging, two streams of fluid will impinge one another resulting in formation of droplets.

The heat conductive block 9 is cylindrical shaped and has a slot for housing the connection tube 6. Once the connection tube 6 is arranged within the slot silicone rubber is filled into the slot to keep the connection tube 6 fixed in the slot and to avoid formation of air pockets within the slot. Such air pockets could result in pockets with high pressure when the temperature of the nozzle 0 increase which could distort the shape of the nozzle if the pressure is high enough.

The nozzle may be equipped with a heating element (not shown), such as a heat wire being electrically heated, e.g. a wire having a resistance sufficient to allow generation of heat when electrical power is supplied. This heating element is typically embedded in one or both nozzle blocks and is typically activated when the nozzle temperature is below an operating temperature.

The heat conductive block 9 is pressed by the spring ring 21 against the nozzle elements 1, 2, 3 in order to assure thermal contact between the block 9 and the elements 1, 2, and 3 so as enabling transferral of heat from the nozzle elements 1, 2, 3 to the outer tube (7) from where convectional heat transfer to the surroundings can take place. The heat conductive block (9) also works as a heat capacitance reducing the effect of temperature spikes.

The ceramic insulation 13 is protected by the outer bushing 14 which also is used for the mounting of the system into the exhaust pipe. The outer bushing 14 is secured between the upper 17 and lower clamp 18.

The nozzle tip A is shielded from the exhaust flow B by mean of the ceramic part 13 that has low heat conductivity compared to other parts of the nozzle made of e.g. metal, such as steel or aluminium. By this only a little flow of exhaust gas passes past the nozzle tip A and thereby transfers heat to the nozzle. Additionally, evaporation of the liquid atomized will also cool the exhaust gas in front of the nozzle tip, however this cooling effect only works while liquid is being atomized. If the nozzle is being blocked and the atomization thereby stopped the temperature on the nozzle block will rise, melt the urea blocking the nozzle thereby assisting in removal of the blockage

In fig. 2 a second embodiment is disclosed. The nozzle 0 comprising a cylindrical pipe 33 through which urea is fed to by the coupling 25 and the strainer 26. Liquefied urea under pressure streams through the channel 33 towards and through the two converging channels 32 whereby two high velocity fluid streams impinges one another to form droplets 34. The nozzle 0 is arranged flush with the interior wall of the exhaust pipe 35.

Surrounding the cylindrical pipe 33 is a heat conductive part 27 made of aluminium. The part 27 besides conducting heat away from the lower part of the nozzle 0 also introduces a heat capacitance which improves resistance to peek heat impacts. Part 27 comprising fins for convection and radiation of heat to the surroundings. The heat capacitance feature may also be incorporated in the other embodiments of the invention and is therefore not limited to an embodiment where the part comprising fins. At the lower part of the nozzle 0 a heat shield 28 is arranged. This heat shield 28 shields the fins of part 27 against heat radiation from the exhaust pipe 35. Between the heat shield 28 and the heat conductive part 27 is provided an air gap 29 (shown to the left in fig. 2) or a ceramic isolation. Alternatively to the heat shield 28 and air gap 29 a ceramic insulation 31 (shown to the right in fig. 2) or in general a material having a low heat conduction property may be applied. In the embodiment shown in fig. 2 a further air gap 29a is provided to avoid direct contact between the heat shield 28 or the ceramic insulation 31 and e.g. the wall of the exhaust pipe and the outer parts of the nozzle. The air gaps 29, 29a, the ceramic insulation 31 or the material having a low heat conduction property surrounds the cylindrical pipe 33 - thus fig. 2 shows two embodiments. When an air gap 29 is provided, the interior surfaces defining the air gap 29 are preferably polished or have a similar low surface roughness in order to minimize heat radiation and heat absorption. The air gap 29 or the ceramic insulation insulates thermally a region of the nozzle in the vicinity of the outlet of the nozzle from heat coming from the surroundings - in this case the wall of the exhaust pipe 35.

Fig. 3 shows a nozzle design with active cooling. As it appears from the figure, the nozzle 0 is very similar to the nozzle shown in fig. 1 except that a cooling unit 36 is applied to the nozzle 0. Thus, the nozzle 0 may be viewed as a passive cooled nozzle design with an active cooling possibility. The nozzle 0 comprising a cooling surface 37 on which surface a sleeve 38 is arranged. The cooling surface 37 as well as the sleeve 38 is both cylindrical and the sleeve 38 is press-fitted on the cooling surface 37 to assure a low heat transfer resistance between the two. The sleeve 38 is connected to the cooling unit 36.

A number of possibilities exist in applying active cooling in embodiments like the one shown schematically in fig. 3. In one preferred embodiment, the sleeve 38 is hollow so as to allow a fluid to be present within the sleeve. The hollow space of the sleeve is in some embodiments

referred to as an evaporation chamber. In such an embodiment, a cooling liquid is fed from the cooling unit 36 to the interior of the sleeve wherein the liquid due to heat transported to it from the nozzle 0 will evaporate causing a cooling effect of the nozzle 0. The evaporated liquid streams to the cooling unit 36 wherein it is condensed into liquid and fed back into the sleeve 38. In order to carry out this evaporation-condensing process the cooling unit 36 comprises a compressor powered by the engine and a condenser. The interior of the cooling unit is then referred to as a condensing chamber.

In another embodiment, the cooling unit 36 together with the sleeve 38 is a Peltier-element which also is powered by engine to which the exhaust pipe is connected. By applying an active cooling design, the amount of cooling is controllable as the cooling may be switched off, the pressure of in the condenser may be changed by changing the operation of the compressor and thereby changing the evaporation / condensing temperatures and the like. Thus, the amount of cooling may be adapted to the need for cooling which may change e.g. based on changes in the surrounding. In fig. 4 a fourth embodiment is disclosed. The fourth embodiment is referred to as a design with selective cooling and several parts of the nozzle 0 are identical to the nozzle 0 shown in fig. 1 - the nozzle 0 of fig. 4 is in general identical with the nozzle of fig. 1 having an evaporation- condensing system added. The evaporation-condensing system comprising an evaporation chamber 39 surrounding an upper part of the nozzle 0, a liquid transportation hose 42, a steam transportation hose 40 and a condensing chamber 41.

Cooling of the nozzle 0 is done by evaporating a liquid streaming into an evaporation chamber 39. Steam is generated by heat transported from the nozzle to the fluid in the evaporation chamber 39, the steam flow through the steam transportation hose 40 to the condensing chamber 41 thereby transporting heat away from the nozzle 0. The condensing chamber 41 is arranged at a location where the temperature is lower than the evaporation temperature of the steam whereby the steam is condensed back into liquid. The thus generated liquid streams back to the evaporation chamber 39 through the liquid transportation hose 42.

The system is orientated accordingly to the gravity so that the cooling chamber 41 is placed higher than the evaporation chamber 39. The system is designed in a way that only steam flows in the steam hose 40. Thus, in contrast to e.g. a heat pipe in which a sliver is used for transporting liquid in the same tube as steam but in opposite direction, the system shown in fig. 4 uses separate hoses for steam and liquid flow. However, the heat pipe system may also be used in the present invention.

By choosing the properties e.g. the evaporation point for the liquid - or in general also the specific fluid - it is possible to design the system so that the nozzle only is cooled after it has reached its working temperature.

This makes it possible for the nozzle quickly to reach its working temperature by means of the heat in the exhaust, and at the same time ensure that the exhaust do not overheat the nozzle which can lead to a blockage.

In the above disclosed embodiments, the liquefied urea is made into droplets by letting two fluid streams impinge one another and this droplet formation principle will be elaborated further below. However, it is to be noticed that although the present description focus on the concept of fig. 5 (discussed in details below) other droplet formation techniques are applicable in connection with and considered within the scope of the present invention.

Fig. 5 shows schematically the overall principle of atomizing a fluid by leading the flow of fluid through two channels arranged so that the exiting fluid streams impinge on one another whereby the fluid is atomized. The fluid is illustrated as being supplied from one fluid line, which typically is pressurized. However, the nozzle may also be used to atomize and at the same time mix two or more different fluids led to the nozzle from different fluid supplies.

With reference to fig. 5 the nozzle 0 comprises an inlet channel 44 through which the fluid to be atomized is fed into the nozzle 0. The inlet channel 44 bifurcate at position a in fig. 5 into two intermediate flow channels 45a and 45b leading the fluid into two distinct outlet flow channels 46a and 46b. The channels 44, 45, 46 constitute flow channels defining a flow path from the inlet 47 of the nozzle 0 to the outlets 48a and 48b of the nozzle. As shown in fig. 5 the outlet flow channels 46a and 46b are continuations of the intermediate flow channels 45a and 45b. The outlet flow channels 46a and 46b are according to the present invention, in general, defined as flow channels providing the streams of fluid directions so as to impinge each other.

A balance between the two fluid streams should exist in order to provide a spray not being lopsided. In order to assure that in embodiments like the one disclosed in fig. 5, the flow resistance between the bifurcation point a and the outlets 48a and 48b and the dimensions thereof respectively is made equally big for the two flow paths. Hereby, the velocity and mass flow for the two fluid streams will become similar, such as equal.

Fluid exiting the outlets 48a and 48b is indicated in fig. 5 with thin lines and it is indicated that the fluid impinges at a distance from the nozzle which impingement results in an atomization as indicated by a fan shaped dotted cloud extending mainly in the down stream direction.

The cross sections of the flow channels within the nozzle may have any shape which may be related to the actual manufacturing process used for making the nozzle. The cross section is preferably circular and the dimensions mentioned in the following then refer to the diameter of the cross section. For other shapes the dimensions refer to a characteristic measure, such as the side length of a quadratic cross section.

The dimensions of the flow channels 44, 45 and 46 are chosen according to the actual use of the nozzle and thereby the amount of fluid to be atomized. In a typical embodiment the cross sections of the channels are circular with a diameter in the order of 0.1 mm. However, the amount of fluid exiting the nozzle will to a large extent be determined by the size of the outlets 48a and 48b and the pressure difference across the outlets 6a and 6b. It is therefore envisaged, that the channels 44, 45 and 46 may have a larger cross section than the outlet and provide an amount of fluid to be atomized being determined by the pressure difference across the outlets 46a and 46b and the cross sectional area thereof. The fluid streams impinging should as discussed above have sufficient kinetic energy in order to be atomized. In some applications of the present invention, the mass flow being atomized will

typically vary at least an order of magnitude such that the minimum mass flow may be as low as 1% of the maximum mass flow. At low mass flow the kinetic energy may be so small that no or only very little atomization occurs. In particular, in case a mass flow of 1% of maximum was supplied continuously to the nozzle the amount of energy per mass unit present in the fluid streams would be less than 0.01% of the amount of energy present in the fluid streams at maximum mass flow. Such a small amount of energy would be insufficient to atomize the fluid. The problem has been solved by the present invention by providing synchronic fluid streams with high flow velocity only intermittently. In such cases it may not be sufficient that the flow resistance between the bifurcation point a and the outlets 48a and 48b and the dimensions thereof respectively is made equally big for the two flow paths. In order to avoid formation of large droplets at start and stop of a pulse of fluid stream one should furthermore seek to assure that the mass of the two fluid strings being confined e.g. between the bifurcation point a and the outlets 48a and 48b are similar, such as identical. If not, one of the fluid strings may accelerate and decelerate faster than the other(s) and a situation where one end of a fluid string is not hit by another fluid string may occur.

Fig. 6 shows schematically an arrangement of a nozzle 0 according to the present invention. An unshielded tip 50 is placed directly in the exhaust flow in to which two streams of liquefied urea

49 flow and impinges one another for formation of droplets. The nozzle tip 50 extend beyond the wall of the exhaust pipe 35. This result in a flow of hot air around the nozzle tip 50 which helps removes liquefied urea derivatives deposited on the surface of the nozzle tip 50. As the tip

50 is exposed to a flow of hot exhaust gasses, this flow may grip liquefied urea and avoid building-up of deposits by removing the liquefied urea before it reaches the surface.

Fig. 7 shows schematically another arrangement of a nozzle 0 according to the present invention. In the arrangement shown in fig. 7, the nozzle tip is flush with the wall of the exhaust pipe 35. On the interior surface of the exhaust pipe 35 a boundary layer will build up, and this boundary layer may in combination with the flow of liquefied urea out of the nozzle have a tendency to shield the tip from heat exposure coming from the heat exhaust gas. However, if a build-up of urea derivatives takes place on the surface of the nozzle tip forming a peak into the exhaust stream, the boundary layer of the exhaust gas will have a tendency to tear this peak off by the viscous forces acting on the surface of the peak.