Ribbed microchannel heat sink and method

Abstract

The application provides a rib-containing micro-channel heat dissipation device and method, and relates to the field of micro-channel heat exchangers; in view of the problems of excessive internal resistance of the micro-channel heat exchanger and poor temperature uniformity in the action area, a plurality of parallel heat exchange channels for the flow of working medium are arranged in the base body, a plurality of groups of basic fins are arranged in the heat exchange channels to divide and guide the heat exchange working medium, meanwhile, the basic fins can also increase the contact area with the working medium to improve the heat exchange effect, so that the arrangement density of the basic fins in the same heat exchange channel is reduced, the flow resistance of the working medium is reduced, and the heat exchange amount of different areas of the heat exchange channel tends to be uniform.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of micro-channel heat exchanger, in particular to a rib-containing micro-channel heat dissipation device and method. BACKGROUND

[0002] Micro-channel is a commonly used device for heat dissipation under high heat flow density, and its principle is to attach to the heat source such as a chip, to take away heat by the cooling medium in the channel through the heat conduction of the substrate. The performance indicators of micro-channel mainly include two aspects of substrate temperature and inlet and outlet pressure drop. The substrate temperature is directly related to the heat dissipation capacity of the micro-channel, and the inlet and outlet pressure drop is related to the pumping power required by the micro-channel, that is, the energy consumption in the operation process of the micro-channel. Therefore, a lower substrate temperature and a lower inlet and outlet pressure drop are required.

[0003] The arrangement form of the fins in the micro-channel affects the flow and heat exchange performance of the working medium inside. Arranging fins in the micro-channel can break the flow boundary layer and the thermal boundary layer, thereby improving the heat exchange effect, but it will also bring a certain increase in pressure drop. At present, the heat exchange effect of the micro-channel is increased by increasing the number and distribution density of the fins, which increases the internal resistance and increases the energy consumption of pumping the working medium. At the same time, due to the increase in the number of fins, the working medium gradually warms up during the flow process, which affects the heat exchange effect of the downstream. The temperature of the heat sink increases along the flow direction of the working medium, resulting in poor temperature uniformity and difficulty in meeting the demand for uniform cooling of the heat source. SUMMARY

[0004] The purpose of the present application is to overcome the defects in the prior art, and to provide a rib-containing micro-channel heat dissipation device and method. A plurality of parallel heat exchange channels for the flow of working medium are arranged in the base body, and a plurality of groups of basic fins are arranged in the heat exchange channels to divide and guide the working medium. At the same time, the fins themselves can also increase the contact area with the working medium to improve the heat exchange effect, thereby reducing the arrangement density of the basic fins in the same heat exchange channel, reducing the flow resistance of the working medium, and making the heat exchange capacity of different regions of the heat exchange channel tend to be uniform.

[0005] The first purpose of the present application is to provide a rib-containing micro-channel heat dissipation device, which adopts the following scheme:

[0006] The base body is provided with a plurality of parallel heat exchange channels, and a plurality of groups of basic fins are arranged in the heat exchange channels in sequence along the flow direction of the working medium. The basic fins are arranged obliquely relative to the axis of the heat exchange channel at the position, the basic fins divide and guide the working medium flowing through, and the inclination offset directions of adjacent groups of basic fins are opposite, so that the heat exchange capacity of different regions of the heat exchange channel tends to be uniform.

[0007] Further, a plurality of flow guide fins are arranged in the heat exchange channels to disturb the working medium flowing through.

[0008] Furthermore, guide ribs are provided on the inner wall on one side of the heat exchange channel inlet, guide ribs are provided on the inner walls on both sides of the heat exchange channel outlet, and guide ribs are provided on the inner walls of the heat exchange channel between some adjacent groups of basic ribs.

[0009] Furthermore, the basic fins include a plurality of independent fins spaced apart in sequence, and the number of independent fins corresponding to the basic fins near the inlet of the heat exchange channel is not less than the number of independent fins corresponding to the basic fins near the outlet of the heat exchange channel.

[0010] Furthermore, the independent ribs have the same orientation as their corresponding basic ribs, and the spacing between adjacent independent ribs in the same group of basic ribs is smaller than the spacing between adjacent groups of basic ribs.

[0011] Furthermore, along the flow direction of the working medium in the heat exchange channel, the end of the basic fin located upstream of the working medium has a pointed structure, and the end located downstream of the working medium has a smooth structure.

[0012] Furthermore, the heat exchange channels in the base are arranged in two stacked layers, with the heat exchange channels in the upper layer corresponding to the heat exchange channels in the lower layer one-to-one, and along the flow direction of the working medium in the heat exchange channels, the basic fins in the heat exchange channels are the same.

[0013] Furthermore, the flow direction of the working medium in the upper heat exchange channel is opposite to the flow direction of the working medium in the lower heat exchange channel.

[0014] A second object of the present invention is to provide a method for operating the ribbed microchannel heat sink, comprising:

[0015] The inlet end of the matrix is ​​connected to an external working fluid supply source, and the working fluid is introduced to the inlet of all heat exchange channels;

[0016] The basic fins in the heat exchange channel guide and divert the working medium, and exchange heat with the heat source at the location of the base body to cool the heat source;

[0017] The working fluid after heat exchange with the heat source in the heat exchange channel is collected at the outlet of the base body and then discharged.

[0018] Furthermore, the pumping power of the working medium is adjusted so that the temperature of the position where the substrate and the heat source are in contact tends to be average.

[0019] Compared with the prior art, the present invention has the following advantages and positive effects:

[0020] (1) In view of the problem of excessive internal resistance and poor temperature uniformity in the action area of the micro-channel heat exchanger, a plurality of parallel heat exchange channels for the flow of working medium are arranged in the base body, a plurality of basic fins are arranged in the heat exchange channel to distribute and guide the heat exchange working medium, and the basic fins themselves can also increase the contact area with the working medium to improve the heat exchange effect, thereby reducing the arrangement density of the basic fins in the same heat exchange channel, reducing the flow resistance of the working medium, and making the heat exchange capacity of different areas of the heat exchange channel uniform.

[0021] (2) The basic fins and the flow guide fins are arranged in each heat exchange channel, which can effectively break the thermal boundary layer and the flow boundary layer, strengthen the heat exchange effect, and at the same time, the basic fins can break the distribution into a plurality of independent fins, promote the disturbance effect when flowing through the basic fin area, and improve the heat exchange capacity.

[0022] (3) The double-layer heat exchange channel structure is adopted, the upper and lower layers are distributed, the upper heat exchange channel and the lower heat exchange channel cooperate to make the heat exchange capacity per unit area stronger, and when the working medium flows in opposite directions in the upper heat exchange channel and the lower heat exchange channel, the high-temperature area and the low-temperature area in the heat exchange channel can be balanced, so that the overall heat exchange capacity has more excellent temperature uniformity. BRIEF DESCRIPTION OF DRAWINGS

[0023] The drawings accompanying the specification of the present application form a part thereof, serve to provide further understanding of the present application, and together with the description of the exemplary embodiments of the present application and the explanation thereof serve to explain the present application, and do not constitute an improper limitation of the present application.

[0024] Figure 1 It is an external schematic view of the rib-containing micro-channel heat dissipation device in embodiments 1 and 2 of the present application.

[0025] Figure 2 It is a schematic view of the heat exchange channel distribution of the rib-containing micro-channel heat dissipation device in embodiments 1 and 2 of the present application.

[0026] Figure 3 It is a schematic view of the heat exchange channel inside the rib-containing micro-channel heat dissipation device in embodiments 1 and 2 of the present application.

[0027] Figure 4 It is a schematic view of the distribution of the fins in embodiments 1 and 2 of the present application.

[0028] Figure 5 It is a streamline diagram of the working medium flowing through the heat exchange channel in embodiments 1 and 2 of the present application.

[0029] Figure 6 It is a comparative schematic view of the rib-containing micro-channel heat dissipation device and other heat exchangers in embodiments 1 and 2 of the present application.

[0030] Among them, 1 substrate inlet, 2 substrate, 3 substrate outlet, 4 heat exchange channel, 2a upper heat exchange channel, 2b lower heat exchange channel, 21 upper plate, 22 base plate, 23 side plate, 24 partition plate, 25 fin group, 251 first basic fin, 252 second basic fin, 253 third basic fin, 254 guide fin, 255 independent fin. DETAILED DESCRIPTION

[0031] Example 1

[0032] In a typical embodiment of the present invention, Figures 1-6 As shown, a ribbed microchannel heat dissipation device is provided.

[0033] Arranging fins in the microchannels of a heat exchanger can improve the heat transfer effect, but it will result in a certain increase in pressure drop. It is necessary to comprehensively consider the flow and heat transfer performance. Most approaches increase the heat transfer time and heat transfer area of ​​the working fluid by increasing the number and density of fins, but without comprehensively considering the flow and heat transfer performance. This leads to increased internal resistance and increased energy consumption for pumping the working fluid. At the same time, there is also the problem of poor temperature uniformity.

[0034] Based on this, a ribbed microchannel heat dissipation device is provided in this embodiment, which has a base 2 with multiple parallel heat exchange channels 4 inside. The base 2 is attached to the heat source, and the heat exchange channels 4 inside the base 2 transport the heat exchange medium to exchange heat with the heat source. Multiple groups of basic ribs are arranged in the heat exchange channel 4 to divert and guide the heat exchange medium. At the same time, the ribs themselves can also increase the contact area with the medium to enhance the heat exchange effect, thereby reducing the arrangement density of the basic fins in the same heat exchange channel 4, reducing the flow resistance of the medium, and making the heat exchange amount in different areas of the heat exchange channel 4 tend to be uniform.

[0035] The ribbed microchannel heat dissipation device in this embodiment will be described in detail below with reference to the accompanying drawings.

[0036] See also Figure 1 The ribbed microchannel heat dissipation device mainly includes a base 2, and multiple heat exchange channels 4 are formed inside the base 2. One end of the heat exchange channel 4 is commonly connected to the base inlet 1, and the other end of the heat exchange channel 4 is commonly connected to the base outlet 3. The heat exchange medium is input into the base 2 from the base inlet 1, and is divided into each heat exchange channel 4. After flowing through the heat exchange channel 4, it is gathered at the base outlet 3 for discharge.

[0037] The internal heat exchange channel 4 is arranged in a double layer, and each heat exchange channel 4 is provided with a fin group 25, such as Figure 2 The arrangement and structure of the fin group 25 are topologically optimized, with the goal of minimizing the inlet and outlet pressure drop of the heat exchange channel 4, and the constraint that the average temperature of the substrate does not exceed the set value, taking into account both flow and heat exchange performance.

[0038] The heat exchange channel 4 after the arrangement of the fin group 25 has excellent comprehensive flow and heat exchange performance, and has the best heat exchange effect under the same inlet and outlet pressure drops, or the pumping power required to achieve the same heat exchange effect is the lowest.

[0039] In the double-layer heat exchange channel 4, the fin groups 25 in the upper heat exchange channel 2a and the lower heat exchange channel 2b can be arranged in the same or opposite directions. Similarly, the working fluid flow direction can be the same or opposite. By configuring according to needs, the heat exchange channel 4 can achieve both excellent temperature uniformity and good heat exchange performance.

[0040] The base 2 includes a base plate 22, an upper plate 21, side plates 23 and partitions 24. The multiple heat exchange channels 4 included in each layer share the base plate 22 and the upper plate 21. The base plate 22 and the upper plate 21 are connected to the side by the side plates 23. The two side plates 23 are separated by multiple partitions 24 to form multiple heat exchange channels 4. Adjacent heat exchange channels 4 are separated by partitions 24.

[0041] like Figure 3 As shown, the upper plate 21 of the lower heat exchange channel 2b serves as the base plate 22 of the upper heat exchange channel 2a, and the structure of the upper heat exchange channel 2a is the same as that of the lower heat exchange channel 2b.

[0042] The upper surface of the base plate 22 serves as the bottom wall of the heat exchange channel 4, the side surface of the side plate 23 or the side surface of the partition plate 24 serves as the side wall of the heat exchange channel 4, and the bottom surface of the upper plate 21 serves as the top wall of the heat exchange channel 4, thereby surrounding and forming the heat exchange channel 4.

[0043] The fin group 25 is arranged in the heat exchange channel 4, the top end of the fin group 25 is connected to the top wall of the heat exchange channel 4, the bottom end of the fin group 25 is connected to the bottom wall of the heat exchange channel 4, and the side surface of the fin group 25 guides and drains the working medium flowing through.

[0044] In response to the need to improve heat exchange performance, the upper heat exchange channel 2a and the lower heat exchange channel 2b have the same direction, and the arrangement of their internal fin groups 25 is the same. The parallel arrangement of the heat exchange channels 4 can improve the heat exchange effect and enhance the heat exchange performance.

[0045] To improve uniform heat transfer across the heat source area, the fin groups 25 within the upper heat exchange channel 2a and the lower heat exchange channel 2b are arranged in opposite directions. The working fluid inlet and outlet of the upper and lower heat exchange channels 2a and 2b are also arranged in opposite directions, resulting in opposite directions of working fluid flow within the upper and lower heat exchange channels 2a and 2b. This countercurrent arrangement balances the high-temperature and low-temperature areas within the heat exchange channel 4, resulting in greater temperature uniformity across the entire heat exchange capacity and improving heat transfer performance.

[0046] likeFigure 3 The fin group 25 includes basic fins and guide fins 254. The heat exchange channel 4 is provided with multiple groups of basic fins arranged in sequence and at intervals, and also has multiple guide fins 254.

[0047] The basic fins are arranged tilted relative to the axis of the heat exchange channel 4 where they are located. The basic fins divert and guide the working fluid flowing through. The tilt offset directions of adjacent groups of basic fins are opposite, so that the heat exchange in different areas of the heat exchange channel 4 tends to be uniform.

[0048] Combine Figure 3 and Figure 4 Taking the arrangement of three groups of basic fins in this embodiment as an example, the three groups of basic fins are respectively arranged on the right side along the flow direction and extending to the right side, the second basic fin 252 along the left side along the flow direction and extending to the left side, and the third basic fin 253 along the right side along the flow direction and extending to the right side. Figure 4 As shown in the top of the figure, the arrangement of the fins has good continuity, which helps to reduce the inlet and outlet pressure drops.

[0049] Near the inlet, the basic fins are arranged close to the left side facing the flow direction. In the middle area, the basic fins are arranged on the right side facing the flow direction. The basic fins near the outlet are arranged on the left side facing the flow direction. This arrangement avoids stratification of flow and temperature distribution and improves the heat exchange performance of the heat exchange channel 4.

[0050] In order to enhance the heat exchange effect, on the basis of arranging basic fins in the heat exchange channel 4, guide fins 254 can also be arranged. A plurality of guide fins 254 are provided in the heat exchange channel 4. The guide fins 254 are arranged in close contact with the inner wall of the heat exchange channel 4 to disturb the working medium flowing through. Figure 4 As shown in the middle, four guide ribs 254 are provided. Guide ribs 254 are provided on the inner wall on one side of the inlet position of the heat exchange channel 4, and guide ribs 254 are provided on the inner walls on both sides of the outlet position of the heat exchange channel 4. Guide ribs 254 are provided on the inner wall of the heat exchange channel 4 between some adjacent groups of basic ribs; the guide ribs 254 are arranged close to the inner wall of the heat exchange channel 4, which has the effect of interrupting the flow boundary layer and the thermal boundary layer and causing fluid disturbance.

[0051] In addition, in order to enhance the heat transfer effect, the basic fins can be interrupted, such as Figure 4 As shown in the lower part, the basic fins include a plurality of independent fins 255 distributed in sequence, and the number of independent fins 255 corresponding to the basic fins near the inlet of the heat exchange channel 4 is not less than the number of independent fins 255 corresponding to the basic fins near the outlet of the heat exchange channel 4.

[0052] In this embodiment, three basic ribs are broken to form a plurality of shorter independent ribs 255. The number of independent ribs 255 formed by breaking the first basic rib 251 should not exceed four, the number of independent ribs 255 formed by breaking the second basic rib 252 should not exceed four, and the number of independent ribs 255 formed by breaking the third basic rib 253 should not exceed two.

[0053] The direction of the independent fin 255 is consistent with that of its corresponding basic fin. The spacing between adjacent independent fins 255 in the same group of basic fins is smaller than the spacing between adjacent groups of basic fins. After the basic fins are split, the streamline diagram of the working medium flowing through the independent fin 255 is as follows: Figure 5 As shown, the working fluid flowing in from the inlet is split into left and right parts after passing through the first independent fin 255, and then flows through the following independent fins 255 in sequence. There are certain gaps between the four independent fins 255, and some working fluid flows from the right area of ​​the first basic fin 251 to the left area, promoting flow mixing. After flowing through the four independent fins 255, the main flow is split into left and right parts again after flowing through the second basic fin 252, and the working fluid originally on the left area flows into the right area, further promoting flow mixing and enhancing the heat exchange effect.

[0054] It is understandable that in actual application, if the simplicity of the processing is pursued, only the first basic fins, the second basic fins and the third basic fins can be arranged. If the heat exchange effect needs to be further enhanced on this basis, the guide fins 254 can be added; if the flow needs to be optimized and the heat exchange effect needs to be enhanced, the basic fins can be broken into several independent fins 255. These three fin arrangement modes can be flexibly selected in actual application.

[0055] With the exception of the guide ribs 254, the basic ribs and the independent ribs 255 that comprise them are all long strips. The specific shapes can be selected and adjusted based on actual needs. The first, second, and third basic ribs 251, 252, and 253 are long strips with a relatively large length and width, while the interrupted independent ribs 255 are long strips with a relatively small length and width. The shape of the guide ribs 254 is not fixed, as long as they are close to the wall and can cause disturbance.

[0056] Along the flow direction of the working fluid in heat exchange channel 4, the end of the basic fin located upstream of the working fluid has a pointed structure, which can have a sharp corner, while the end located downstream of the working fluid has a smooth structure, which can have a rounded corner. When the cooling working fluid flows through fin assembly 25, the flow boundary layer and the thermal boundary layer are interrupted and redeveloped, promoting heat exchange between heat exchange channel 4 and the cooling working fluid.

[0057] Correspondingly, the heat exchange channels 4 in the base 2 are arranged in two stacked layers, with the upper heat exchange channels 2a corresponding one to the lower heat exchange channels 2b. Along the flow direction of the working medium in the heat exchange channels 4, the basic fins in the heat exchange channels 4 are the same; in order to improve the temperature uniformity of the heat exchange area, the flow direction of the working medium in the upper heat exchange channels 2a is opposite to the flow direction of the working medium in the lower heat exchange channels 2b.

[0058] Below, a set of dimensions and specifications of the ribbed microchannel heat sink of this embodiment are given.

[0059] The heat exchange channel 4 inside the ribbed microchannel heat sink in this embodiment is a microchannel structure, and the equivalent diameter of each heat exchange channel 4 is not greater than 1 mm; the length of each heat exchange channel 4 is not greater than 15 mm; and the width of the heat exchange channel 4 is not greater than 1 mm.

[0060] Correspondingly, the lengths of the first and second ribs are both less than 6 mm, the length of the third rib is less than 3 mm, and the length of the independent rib 255 after being broken is less than 2 mm, the length of the guide rib 254 is less than 1.5 mm, and the thickness of all ribs is less than 1 / 3 of the width of the heat exchange channel 4.

[0061] In addition, the shapes of the fins included in the fin group 25 are consistent in the height direction; the number of heat exchange channels 4 included in the upper heat exchange channel 4 and the lower heat exchange channel 2b is greater than or equal to eight, depending on the specific application conditions.

[0062] In other embodiments, when the application space of the ribbed microchannel heat dissipation device is not limited, if it is necessary to enhance the heat exchange performance, the number of heat exchange channels 4 can be increased on the basis of the double-layer heat exchange channel 4, and a multi-layer heat exchange channel 4 structure can be adopted. In order to ensure the improvement of the heat dissipation effect, the heat exchange channel 4 does not exceed four layers.

[0063] Figure 6 The flow and heat transfer performance of the ribbed microchannel heat sink proposed in this embodiment are verified. The ribbed microchannel heat sink proposed in this embodiment is compared with a single channel without ribs, a single channel with square ribs of equal rib area, and a single channel with circular ribs of equal rib area. The horizontal axis is the pumping power, and the vertical axis is the average temperature of the microchannel substrate. In the application process of microchannel radiators, low substrate temperature and low pumping power are pursued, so the curve close to the origin of the coordinate indicates that the comprehensive performance of flow and heat transfer of the microchannel is better. It has been verified that the ribbed microchannel heat sink proposed in this embodiment has the best comprehensive performance of flow and heat transfer.

[0064] Example 2

[0065] In another typical embodiment of the present invention, Figures 1-6 As shown, a working method using a ribbed microchannel heat dissipation device is provided.

[0066] In combination Figures 1-6 The working method includes the following steps:

[0067] The base inlet 1 is connected to an external working medium supply source, and the working medium is introduced into all the heat exchange channels 4;

[0068] The basic fins in the heat exchange channels 4 guide and distribute the working medium, and exchange heat with the heat source arranged at the position of the base 2, thereby cooling the heat source;

[0069] The working medium after the heat exchange with the heat source in the heat exchange channels 4 is collected and discharged after the base outlet 3.

[0070] The working method is described in detail in combination with the drawings and the embodiment 1.

[0071] After the working medium flows into the base 2 through the base inlet 1, it is distributed into multiple heat exchange channels 4 corresponding to the upper heat exchange channels 2a and the lower heat exchange channels 2b. The heat from the heat source is directly absorbed by the cooling working medium after being conducted by the base plate 22 of the heat exchange channels 4, and is transmitted to the out-of-plane and then absorbed by the cooling working medium after being conducted by the base plate 22 and the fin group 25 in different heat exchange channels 4. The heated cooling working medium flows out through the base outlet 3, is cooled and discharged externally, and then flows into the base inlet 1 again to form a cycle.

[0072] The above only describes the preferred embodiments of the present application and is not used to limit the present application. For those skilled in the art, the present application can have various modifications and changes. Any modification, equivalent replacement, improvement, etc. within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A ribbed microchannel heat dissipation device, characterized in that: The heat exchanger comprises a base body, wherein a plurality of parallel heat exchange channels are provided in the base body. Three groups of basic fins are arranged in sequence and spaced apart along the flow direction of the working medium in the heat exchange channel. The basic fins are arranged obliquely relative to the axis of the heat exchange channel in which they are located. The basic fins divide and guide the working medium flowing through the heat exchange channel. The inclination and offset directions of adjacent groups of basic fins are opposite, so that the heat exchange rate in different areas of the heat exchange channel tends to be uniform. The basic ribs include a plurality of independent ribs spaced apart in sequence, and the independent ribs have the same orientation as their corresponding basic ribs; the basic ribs and the independent ribs are both in the shape of long strips; Along the flow direction of the working medium in the heat exchange channel, the end of the basic fin located upstream of the working medium is a pointed structure, and the end located downstream of the working medium is a smooth structure; A plurality of guide fins are provided in the heat exchange channel, and the guide fins are arranged in close contact with the inner wall of the heat exchange channel to disturb the working medium flowing through; The number of independent fins corresponding to the basic fins near the inlet of the heat exchange channel is not less than the number of independent fins corresponding to the basic fins near the outlet of the heat exchange channel; The distance between adjacent independent fins in the same group of basic fins is smaller than the distance between adjacent groups of basic fins.

2. The ribbed microchannel heat sink according to claim 1, wherein: Guide ribs are provided on the inner wall on one side of the heat exchange channel inlet, guide ribs are provided on the inner walls on both sides of the heat exchange channel outlet, and guide ribs are provided on the inner walls of the heat exchange channel between some adjacent groups of basic ribs.

3. The ribbed microchannel heat sink according to claim 1, wherein: The heat exchange channels in the base are arranged in two stacked layers, with the heat exchange channels in the upper layer corresponding to the heat exchange channels in the lower layer one by one. Along the flow direction of the working medium in the heat exchange channels, the basic fins in the heat exchange channels are the same.

4. The ribbed microchannel heat sink according to claim 3, wherein: The flow direction of the working medium in the upper heat exchange channel is opposite to the flow direction of the working medium in the lower heat exchange channel.

5. A method for operating the ribbed microchannel heat dissipation device according to any one of claims 1 to 4, characterized in that: include: The inlet end of the matrix is ​​connected to an external working fluid supply source, and the working fluid is introduced to the inlet of all heat exchange channels; The basic fins in the heat exchange channel guide and divert the working medium, and exchange heat with the heat source at the location of the base body to cool the heat source; The working fluid after heat exchange with the heat source in the heat exchange channel is collected at the outlet of the base body and then discharged.

6. The working method according to claim 5, characterized in that: Adjust the pumping power of the working fluid to make the temperature of the substrate and the heat source contact position tend to be uniform.

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