JPS6027917B2 - Heat exchanger tubes in the evaporator of compression refrigeration cycles for air conditioning - Google Patents

Description

[Detailed Description of the Invention] [Field of Application of the Invention] The present invention relates to heat transfer tubes used in evaporators of compression refrigeration cycles for air conditioning such as air conditioners and refrigerators that use fluorocarbon-based cold soot. be.

[Background of the invention]

The main components of compression refrigeration cycles for air conditioning such as air conditioners and refrigerators that use fluorocarbon-based cold soot are compressors, condensers,
an expansion mechanism and an evaporator.

Conventionally, heat transfer tubes of the evaporator include finned tubes in which fins are provided or grooved tubes in which grooves are provided. These attempts were to increase the heat transfer area within the tube by providing fins or grooves, and to increase the turbulence of the flow within the tube to improve the heat transfer coefficient. Therefore, it has been considered necessary to increase the fin height or groove depth to a certain extent or more. Such a heat exchanger tube inevitably involves a large pressure loss. A large pressure loss means that the pump power of the compressor (pump means) for circulating the medium increases significantly, and in terms of the refrigeration cycle, it causes changes in the condensing temperature and evaporation temperature, which reduces heat transfer. This has precluded the use of this type of heat exchanger tube because it offsets the performance improvement due to the increased heat exchange rate. [Object of the Invention] The object of the invention is to provide a heat exchanger tube for an evaporator that has a high heat transfer coefficient and a pressure loss comparable to that of a smooth tube in the process of evaporation of cold fluorocarbon soot flowing inside the tube. .

[Summary of the invention]

The first feature of the invention is that the depth h of the spiral groove formed on the inner surface of the evaporator heat exchanger tube is 0.02 to 0.2 ribs, and the pitch p between adjacent grooves is 0.1 to 0. On the .5 side, the ratio of the groove depth h to the groove width w (h/w) is U-shaped, which is larger than 0.4, thereby increasing the heat transfer coefficient and making the pressure loss comparable to that of a smooth pipe. be.

The feature of the second invention is that the feature of the first invention is added that the inclination angle 8 with respect to the tube axis is set to 4o to 1y. This improves the rate.

, about Tsune's ′, ・Bell.

Groove depth h'; In compression refrigeration cycles for air conditioning, generally cold soot is R-
Freon-based cold soot such as 22, R-12 is used and the circulating flow rate (
The weight flow rate per heat transfer tube is 30 to 80k9.
/Hr.

The refrigerant liquid is depressurized by the expansion mechanism before flowing into the evaporator, so when it flows into the heat transfer tube of the evaporator, it is in a two-phase flow state of vapor and liquid. Because it is forced into circulation, it flows at a considerable speed. Therefore, in the cooling chamber inside the pipe, the liquid flows while clinging to the inner wall surface of the pipe, and the steam flows in the center of the pipe in an annular flow or a semi-annular flow similar to an annular flow. On the other hand, the refrigerant that flows into the heat exchanger tube in a two-phase flow evaporates while taking heat from outside the tube while flowing inside the tube, so the liquid film thickness of the soot liquid in the annular flow state Although the liquid film thickness gradually becomes thinner, the average liquid film thickness is about 0.2 degrees.

When the groove depth (fin height) is made larger than the average liquid film thickness, the fin tips protrude from the cold soot liquid film, and this protruding portion does not directly and effectively contribute to heat exchange with the soot liquid. Therefore, it is desirable that the groove depth be equal to or less than the average liquid film thickness of the refrigerant liquid in the pipe.

As described above, the refrigeration cycle to which the present invention is applied is one in which refrigerant is forced to circulate, and the smaller the pressure loss during circulation, the lower the pump power of the compressor can be.

As shown in FIG. 6, the experimental results show that the pressure loss is comparable to that of a smooth pipe when the groove depth is up to 02 ribs, and when the groove depth is 0.
Pressure loss increases beyond Cape 2. The reason for this is that the flow velocity at the fin tip exceeds the average liquid film thickness.
This is because it protrudes into cold soot vapor. As mentioned above, the upper limit value of the groove depth of 0.2 is set to a value comparable to the average liquid film thickness of the cold soot liquid inside the pipe, and a value of the groove depth at which the pressure loss is comparable to that of a smooth pipe. It is.

Regarding the lower limit of groove depth h: When the liquid evaporates into only vapor, the flow of refrigerant in the heat transfer tube in the evaporator is such that the flow velocity is high in the center of the tube, and as it approaches the tube wall. The flow velocity becomes smaller.

The region where the flow velocity is high at the center of the pipe is called the turbulent region, and the region near the pipe wall where the flow velocity rapidly decreases is called the laminar bottom layer.
Even if the flow velocity in the turbulent region is high, the flow velocity in the laminar bottom layer is small, so even if there is refrigerant liquid in the groove, the flow of soot vapor in the laminar bottom layer can be used in this liquid. It is not possible to generate vortices.

Therefore, the groove depth needs to be deeper than the thickness of the laminar bottom layer. The depth of the laminar flow bottom layer can be determined by the following formula [1], and this '1' formula is published on page 104 of "Introduction to Heat Transfer" written by Yoshiro Koto and published by Yokendo Co., Ltd. ing.

￥-Kitai ．・・・． ...'11 In the m formula, r is the radius of the heat exchanger tube (half the inner diameter), Re
d is the Shinolds number, and can be determined from the following equation '2'' using the average flow velocity Um of cold soot steam, the inner lining of the heat transfer tube d, and the kinematic viscosity coefficient of soot steam.

Red=U house・d...'21 Typical conditions currently used in the general formula for compression refrigeration cycles for air conditioning (cold soot R-22, evaporation temperature 000, evaporation pressure 4k9
/ block G, d = 1 Qianqi, Um = 7.2 /sec) and (2
) calculation, the value of Red is Red=1.3
×1 pi.

Substituting this value into the {1} formula, we get 6c- 123 r (1.3×1 bi)7/s Electric shock:=o●oo4.

Since the radius r of the heat exchanger tube is on the 5 side, the thickness 6c of the laminar flow bottom layer at this time is found to be 6'=0.02 ribs.

Generally, in a compression type refrigeration cycle, the inner diameter d of the heat transfer tube is 7~
12 ribs, average flow velocity Um of cold soot vapor is 7 to lmh/sec
From these values, the thickness of the laminar bottom layer 6 is calculated as above.
If c is calculated, it will be 0.016 to 0.021 willow, and the average thickness of the laminar bottom layer will be 0.02 ribs. As mentioned above, the lower limit value of 0.02 ribs for the groove depth h is determined from the thickness of the laminar bottom layer.

Groove shape (ratio of groove depth h to groove width w of U-shaped groove h/
w and the apex angle of the V-shaped groove): To improve the heat transfer performance, it is necessary to increase the heat transfer area and disturb the flow of the cooling butterfly, especially by utilizing the flow of refrigerant vapor in the center of the pipe. It is effective to generate vortices in the refrigerant liquid in the grooves.

When the h/w value of the U-shaped groove is gradually increased from zero, the vortices that occur in the groove are formed at the groove depth as follows.
The ratio of the depth of the cold soot liquid in the U-shaped groove to the length of the cold soot liquid in the groove in contact with the refrigerant vapor, that is, the groove depth h and the groove width w
This is when the ratio (h/w) is 0.4. h/w is 04
If it is smaller, the length of contact with the liner vapor will be too large compared to the depth of the liquid, and the flow of the liner vapor will cause the cold soot in the groove to flow over the side wall of the groove and into the adjacent groove. The size of the disaster becomes much smaller than the depth of the trench.

When h/w is gradually increased from 0.4, the length of contact between the cold soot liquid in the groove and the refrigerant vapor becomes smaller, so the liner liquid does not go over the side walls of the groove, and the groove depth increases. A full vortex is stably generated, and when h/w is set to 1 or more, a vortex is stably generated throughout the groove width. A vortex formed over the full width of the groove or the full depth of the groove is hereinafter simply referred to as the largest vortex. In addition, in the case of a V-shaped groove, since the bottom of the groove is tapered, the h/w value is slightly larger than that of a U-shaped groove (approximately 0.5), creating the largest vortex in the refrigerant liquid in the groove. occurs. On the other hand, in the case of a V-shaped groove, the groove depth h versus the groove width w can be expressed using the apex angle, and the above h/w=
If the apex angle y is calculated from 0.5, y=900. Therefore, the shape of the groove is determined based on the limit of generating the maximum vortex in the cooling liquid in the groove by utilizing the flow of soot vapor in the center of the pipe. , vertical angle y≦90o
and in the case of a U-shaped groove, h/wZO. 4.

Regarding the upper limit of the pitch p of the groove: When forming a V-shaped groove with a groove depth of 0.2 ribs and an apex angle of 90'', the width of the groove at this time is the widest point (referred to as the maximum width). It will have four legs.

If the maximum value of this V-shaped groove and the groove pitch are the same, V
Among those in which the apex angle of the grooves is 900, the one with the densest V-shaped grooves is obtained, and the heat transfer area is the largest. In addition, when actually forming a V-shaped groove, it is recommended to provide a slight distance between one groove and the adjacent groove in order to manufacture the forming tool, facilitate the forming process, and maintain processing accuracy. It's advantageous.

In consideration of such practicality, the upper limit value of the groove pitch of the V-shaped groove is set to 0.5, which is slightly larger than the maximum width of 0.4. Considering the value of h/w at which the vortex generated in the refrigerant liquid in the grooves is at its maximum, and considering that the number of grooves should be as large as possible in order to increase the heat transfer area, the groove depth is 0.2. The width of the groove at the maximum of 4 when it is a leg is about 0.5 side,
This value is taken as the upper limit value of the groove pitch p of the U-shaped groove. Regarding the lower limit of the pitch p of the grooves: When soot liquid is flowed into a pipe having grooves separated by fins, the flow that has separated the fins will return to the pipe wall at a downstream position of 5 to 1 pitch of the fin height. It has been reported that a high heat transfer coefficient was observed at this reattachment point.

Considering this and the aforementioned aim of increasing the heat transfer area as much as possible to improve the heat transfer coefficient, we adopted the minimum value of the distance from the fin to the reattachment point, and set the lower limit of the groove depth to this value. The value multiplied by (5×0.02=0.1 ribs) is taken as the lower limit value of the groove pitch p in the V-shaped groove and the U-shaped groove. Regarding the inclination angle 8 with respect to the tube axis; heat transfer coefficient is further improved by making the cold soot liquid swirl in each groove while flowing along the grooves by utilizing the flow of cold soot vapor in the center of the tube. We have found that this method is effective in

As shown in Figure 7, the results of experiments conducted under typical conditions commonly used in compression refrigeration cycles show that when the cheek angle 8 to the tube axis ranges from 4o to 15o, the heat transfer area due to groove formation is The heat transfer coefficient has improved by more than the increase in .

This is because the flow of refrigerant vapor is faster than the flow of cold soot liquid and flows at an angle of 40 to 15 degrees with the groove, so the refrigerant liquid in the groove receives rotational force and force moving along the groove through the surface of the liquid. It is to receive. [Embodiments of the Invention] Examples of the invention will be described below with reference to FIGS. 1 to 9.

As shown in FIGS. 1 and 2, a spiral groove 2 having a shape close to a V-shape is provided on the inner wall surface of the smooth tube 1, and the depth h of this groove 2 is set at a pitch of 0.02 to 0.2 ca. p from 0.1 side to 0.
5. The cross-sectional shape of the sail and groove (cross section perpendicular to the groove) is as shown in Figures 3 and 4. In the case of a V-shaped groove, the apex angle y is an acute angle (90 degrees or less), and in the case of a U-shaped groove, makes the ratio h/w of depth h to width w greater than 0.4.

This reduces the pitch between adjacent grooves, increases the heat transfer area, and generates vortices in the refrigerant liquid in each groove by utilizing the flow of cold soot vapor at the center of the tube. Also,
The inclination angle 8 of the groove with respect to the tube axis is set to 40 to 15 degrees to improve the heat transfer coefficient more than the increase in heat transfer area due to the groove formation.

FIGS. 5 and 6 show the results of experiments conducted on conventional smooth tubes, finned or grooved tubes, and steel heat exchanger tubes according to the present invention under the following conditions.

Cold soot used: R-22
Pressure of boiling liquid...4k9/KiG
Boiling liquid flow rate (weight)...43k9/
Heat flow rate with added Hr...5000kc
al/Hr The solid line a in Figure 5 shows the relationship between the tube fin height or groove depth and the heat transfer coefficient, and the horizontal axis represents the tube inner fin height or groove depth h, that is, the surface roughness. Take
The vertical axis represents the ratio (Q/Qo) between the heat transfer coefficient Q of the heat transfer tube of the present invention and the heat transfer coefficient Qo of the smooth tube.

The ○ marks are the actual measured values obtained. According to this result, when the height of the fins in the tube or the depth h of the grooves is 0.2 or less, a boiling heat transfer coefficient several times that of a smooth tube can be obtained. Note that the broken line b indicates the rate of increase in the heat transfer area within the tube, and conventionally it was thought that only the rate of improvement in the heat transfer coefficient as indicated by the broken line b could be obtained. Figure 6 shows the relationship between the height of the pipe fins or the depth of the grooves and the pressure loss, where the horizontal axis represents the height of the pipe fins or the depth h of the grooves, that is, the surface roughness, and the vertical axis represents the Ratio of pressure loss △p in heat transfer tubes to pressure loss △po in smooth tubes (△
p/△po), the pressure loss gradually increases when the pipe fin height or groove depth h is 0.2 or more, but when the groove depth h is less than 0.2, the pressure loss gradually increases. The pressure loss is almost the same as that of a smooth tube, and the heat transfer coefficient is higher than that of a smooth tube.

Figure 7 shows a V-shaped spiral groove with a depth of 0.2 legs and a pitch of 0.
．． The results of experiments are shown for aluminum heat exchanger tubes in which the inclination angle 8 with respect to the tube axis was varied in the range of oo to 75 degrees, with 5 plates and 11.2 willows being constant.

The horizontal axis represents the angle 3 (hereinafter simply referred to as the angle 8) with respect to the tube axis, and the vertical axis represents the heat transfer coefficient Q and the pressure loss Δp. The pressure loss Δp is almost constant without being affected by the head angle 8, whereas the heat transfer coefficient Q varies greatly depending on the head angle 8.

The values for the smooth tube are shown on the left as a basis for comparison. The heat transfer coefficient Q is smaller when the inclination angle 8 is 00, that is, when the vertical groove is parallel to the tube axis than when it is a smooth tube, and as the groove angle 3 increases, it gradually increases, and reaches a peak around the groove angle 8 = 7o. , it decreases again, reaches its lowest value around 8=450, and then begins to increase again. Therefore, the optimum tilt angle 8 is around 7o. In this invention, the inner surface area of the heat transfer tube is
By cutting the V-shaped groove into a spiral shape, it is approximately 3 times smaller than a smooth pipe.
It has increased by 5%.

Considering that the heat transfer coefficient increases by the same amount as the heat transfer area increases by 35%, the heat transfer coefficient increases to the dotted line A in FIG.

The range of the inclination angle 8 in which the heat transfer coefficient of the smooth tube is 35% or more, that is, the range in which the heat transfer coefficient is higher than the dotted line A is the square angle B=4o to 1y. In this range, the heat transfer coefficient becomes higher than the increase in surface area due to groove formation. FIG. 8 is a diagram showing changes in the heat transfer coefficient Q due to changes in the apex angle y of the V-shaped groove.

The vertical axis of this figure shows the heat transfer coefficient Q (kcal/reh.0), and the horizontal axis shows the medium flow rate Gr (k9/h).

Curve o is a smooth tube, curve y6o is apex angle y = 900, curve y6o is apex angle y = 600, curve y3o is apex angle y = 30
It shows the heat transfer coefficient in the case of o. It can be seen from this figure that the smaller the apex angle 8 of the V-shaped groove, the higher the heat transfer coefficient. At this time, when the apex angle y decreases by 4.times., the groove pitch p also decreases. FIG. 9 is a diagram showing changes in heat transfer coefficient Q due to changes in width W of a U-shaped groove close to a U-shape.

In this figure, the vertical axis shows the heat transfer coefficient Q (kcal/h°C), and the axis shows the cooling flow rate Gr (kcal/h). Curve Wo is a smooth pipe, curve W2 is a groove width w = 0.9 ribs,
The curve W2 has a groove width w=0.55 legs, and the curve W3 has a groove width w=0.
．． This is the case with 25 ribs. It can be seen from this figure that the smaller the width w of the U-shaped groove, the higher the heat transfer coefficient. Furthermore, pitch p
is formed approximately twice the width w. The reason why the heat transfer coefficient improves more than the increase in surface area as shown in the above experimental results is as follows.

In other words, if you make fine grooves with a groove depth h of 0.02 degrees, a pitch p of 0.1 to 0.5 degrees, and a roughness angle P of 4o to 15o, the boiling (refrigerant) liquid will flow through the center of the gas. While forming a film, it receives a strong rotational force in the direction of the groove and flows at a high speed in the direction of the groove.

Riding on this large swirling flow, the boiling liquid inside the groove flows in the direction of the groove, and because the flow direction of the refrigerant vapor and the angular direction of the groove are slightly different, it makes a small vortex movement, resulting in secondary It forms a swirling vortex. As a result of this large swirling flow and small vortex motion, the heat transfer coefficient is most effective between the boiling (cold soot) liquid and the tube wall, and high heat transfer performance is achieved. The reason why the pressure loss is about the same as that of conventional smooth tubes is that the groove depth is about the same as the average liquid film thickness of the cold butterfly inside the tube, which prevents boiling (cold butterfly) liquid from adhering to the tube wall of the heat transfer tube. This is because when the refrigerant liquid flows, most of the fine grooves are covered and smoothed, creating a flow substantially similar to that of a smooth pipe.

〔Effect of the invention〕

As explained above, according to the first invention, a heat transfer tube in an evaporator of a compression refrigeration cycle for air conditioning, which has a higher heat transfer coefficient than a smooth tube and a pressure loss as small as a smooth tube, can be used. According to the second invention, in addition to the effects of the first invention, there is provided a heat transfer tube in an evaporator of a compression refrigeration cycle for air conditioning, in which the heat transfer coefficient is higher than the increase in heat transfer area due to groove formation. can be provided.

[Brief explanation of the drawing]

Fig. 1 is a sectional view of an example of a heat exchanger tube according to the present invention, Fig. 2 is a sectional view taken along the 0-Ro line in Fig. 1, and Fig. 3 is an enlarged view of essential parts showing an example of a V-shaped groove. The sectional view, Figure 4, is U
FIGS. 5 to 9, which are enlarged cross-sectional views of essential parts showing an example of the shape groove, are diagrams showing the results of performance experiments of the heat exchanger tube according to the present invention. 1... Heat exchanger tube, 2... Groove. Figure 1 Figure 2 Figure 5 Figure 3 Figure 6 Figure 4 Figure 7 Figure 8 Figure 9

Claims (1)

[Claims] 1. An evaporation system for an air conditioning compression refrigeration cycle in which a phase-changing fluorocarbon refrigerant is flowed through a pipe in a two-phase flow state by a pump means, and the pipe has a spiral groove on its inner wall. In the heat exchanger tube in the container, the spiral groove has a depth of 0.02 m.
m~0.2mm, pitch between adjacent grooves is 0.1m
In an evaporator for a compression refrigeration cycle for air conditioning, the cross-sectional shape perpendicular to the groove is formed in a U-shape with a ratio of groove depth to groove width greater than 0.4. heat exchanger tube. 2. In a heat transfer tube in an evaporator of a compression refrigeration cycle for air conditioning, in which a phase-changing fluorocarbon refrigerant is flowed through the tube in a two-phase flow state by a pump means, and the tube has a spiral groove on the inner wall. The depth of the spiral groove is 0.02m.
m~0.2mm, pitch between adjacent grooves is 0.1m
m~0.5mm, inclination angle to the tube axis is 4°~15°
A heat exchanger tube in an evaporator of a compression refrigeration cycle for air conditioning, characterized in that the cross-sectional shape perpendicular to the groove is formed in a U-shape with a groove depth to groove width ratio of greater than 0.4.