Formation mechanism for planar structure consisting of circumferential spiral grooves.
As the power of micro-optoelectronic devices increases, so does the heat flow density. Therefore, heat transfer technology needs to be upgraded to achieve higher thermal performance and energy efficiency. The effectiveness of phase-change heat transfer is times greater than that of conventional heat exchange through solid metals . This enhancement in efficiency is realized largely through the heat absorption and release in the cyclic evaporation-condensation process of a working medium. Under the same conditions, the performance of a finned heat transfer surface is 50-180% better than that of a dull surface. Finned heat transfer surfaces, constructed by adding fins to plain surfaces for better heat transfer performance, are referred to as functional surface structures for heat transfer and can be used in phase-change heat transfer devices .
Enhanced boiling structures such as grooves, fins, and porous surfaces are fabricated by mechanical methods , electric discharge methods , spraying , sintering methods , etc. Among these methods, mechanical processing methods have attracted more attention because of their efficiency and low cost [7-9]. Micro- and nanostructures on a dull surface increase the number of nucleation sites, reduce the onset of nucleate boiling, and change a single-phase region into a two-phase region, all of which contribute to nucleate boiling . Further studies have shown that the geometrical parameters and configurations; fluid type; and experimental conditions such as the heat flux, mass flux, pressure, and liquid sub cooling have a considerable effect on the extent of boiling enhancement. Qi Zhang  studied the performance of poly-V grooved aluminum pulleys by performing experiments on a power spinning lathe with optimized process parameters. M. Ghajar  proposed a new model to simulate a micro-loop heat pipe with rectangular capillary grooves. The results showed that geometry optimization, particularly, that of the slot dimensions, is necessary to design an improved micro loop heat pipe device. Jung-Chou Hung  created helical grooves with a depth of 4.9 [micro]m and a machining time of 3 s in a light surface by electrochemical micromachining. Guilian Wang  fabricated micro scale ribs and grooves on the heated wall of a micro channel heat sink using different surface-micromachining microelectromechanical systems to improve the heat transfer rate.
Al-Mokhtar O. Mohamed  experimentally designed a grinding-wheel grooving system to create helically shaped circumferential grooves on the grinding wheel surface. The resulting maximum differences in the slot width and depth were found to be 0.015 and 0.013 mm, respectively, for ten consecutively cut grooves.
In this context, a planar enhanced boiling structure made of circumferential spiral V-shaped grooves is proposed in this paper. A series of analyses are carried out on the effect of the ploughing-extrusion speed, rotational speed, and cutting feed on the structure of V-shaped grooves, fin height, and ratio of the fin height to the ploughing-extrusion depth. The relationship among the ploughing-extrusion speed, cutting feed, and processing time as well as the relationship between the cutting feed and processing time are analyzed.
2. Experimental for forming circumferential spiral microgrooves
Before processing, to create circumferential spiral microgrooves on the end surface, the solid work piece (copper plate) needs to be clamped in the chuck center lathe. The cutting tool is installed parallel to the axis of the work piece, allowing the instrument to advance in the radial direction of the work piece. When the spindle of the lathe rotates the work piece, the cutting tool produces a cutting feed to form a boiling structure of spiral micro-grooves in the interior surface of the evaporation end of the workpiece. Fig. 1, a shows a schematic diagram illustrating the forming process of the structure.
The cutting tool, having dimensions of 5 mm X 5 mm X 40 mm, is made of W18Cr4V high-speed steel. The tool has a cutting edge, central extrusion surface [A.sub.r], subsidiary extrusion surface [A'.sub.r], main forming surface [A.sub.beta], and subsidiary forming surface [A'.sub.beta]. As shown in Fig. 1, b, [gamma.sub.0] is the primary extrusion angle, [gamma'.sub.0] is the subsidiary extrusion angle, [beta] is the central forming angle, and [beta] is the branch forming the angle, with the cutting edge at the front side.
The cutting edge first plows the metal, pushing the metal toward the main and subsidiary extrusion surfaces of the cutter, causes the main extrusion surface to cut into the metal and force it in the direction of the central forming angle. Consequently, grooves are created under the force exerted along this cutting edge.
[FIGURE 1 OMITTED]
3. Results and discussion
3.1. Effect of cutting feed [Florin] or [Guilder][.sub.p] on microgroove shape
A test is carried out to determine the effect of the cutting feed [Florin] or [Guilder][.sub.p] on the ploughing-extrusion depth [a.sub.p], using a cutting tool under the following conditions: [rho.sub.0] = 30[degrees], [rho'.sub.2] = 10[degrees], [beta] = 15[degrees], and [beta]' = 5[degrees]. As is known from experiment, the pitch of the spiral grooves [d.sub.p] is directly proportionate to the cutting feed [Florin] or [Guilder][.sub.p]. It is found that the fin is 0.41, 0.56, and 0.59 mm high when [Florin] or [Guilder][.sub.p] is 0.75, 1.24, and 1.50 mm, respectively, when [a.sub.p] is maintained at 0.30 mm (Fig. 2, a - c). That is, when [a.sub.p] remains constant, the fin height grows with [Florin] or [Guilder][.sub.p]. This is because in the groove-forming process when [Florin] or [Guilder][.sub.p] is relatively small, the extrusion effect is dominant. As [Florin] or [Guilder][.sub.p] increases, part of the extruded metal is drawn out of the groove, rising above the original surface. Therefore, the cutter acts on the fin at a level that is equal to the height of the fin formed immediately before, adding a sharp edge to the current fin. In this way, the fin height increases continuously until the cutting produces an effect comparable to that of extrusion on the fin.
[FIGURE 2 OMITTED]
With a further increase in [Florin] or [Guilder][.sub.p], the formation of the current groove has no influence on the previous groove. Because of extrusion, parts of the metal at the bottom of the groove are squeezed out and accumulate along the edges, remaining above the original metal surface, constituting a secondary slot. At this point, the cutting tool has no effect on the fin, and the fin height cannot increase further. As [Florin] or [Guilder][.sub.p] reaches 1.96 mm, the fin is 0.54 mm above the groove bottom, as shown in Fig. 2, d. The test also indicates that the width of the secondary slot increases with [Florin] or [Guilder][.sub.p].
A ploughing-extrusion test is conducted at different ploughing-extrusion depths and cutting feed to evaluate their effect on the fin height Fig. 3. The figure indicates that the fin height reaches its peak when [Florin] or [Guilder][.sub.p] is increased to a critical value. For a constant value of [a.sub.p], the fin height increases, but it declines gradually as [Florin] or [Guilder][.sub.p] increases. In addition, the maximum fin heights vary with different values of [a.sub.p]. When [Florin] or [Guilder][.sub.p] is kept unchanged, the fin height increases slowly as [a.sub.p] increases. A small increase in [Florin] or [Guilder][.sub.p] gives a slight increase in the fin height. As [a.sub.p] increases to 0.5 mm, the cutter cuts deep into the metal, which generates a large force during the forming process, causing the lathe to vibrate so excessively that it ceases to operate.
[FIGURE 3 OMITTED]
3.2. Effect of ploughing-extrusion depth [a.sub.p] on microgroove shape
[FIGURE 4 OMITTED]
Fig. 4 shows the shapes of grooves for different ploughing-extrusion depths when the cutting feed is kept constant. The figure indicates that the microgroove depth increases with [a.sub.p] increases. Because of the extrusion effect of the subsidiary extrusion surface [A'.sub.gamma] and the trimming effect of the branch forming surface [A'.sub.beta], the excess metal left between grooves begins to bend toward the normal direction of [A'.sub.beta]. As seen from Fig. 4, a and b, when [a.sub.p] is 0.1 and 0.20 mm, the fin height reaches 0.2 and 0.25 mm, respectively, with [Florin] or [Guilder][.sub.p] being kept constant at 0.50 mm. Because the cutting feed is larger than [a.sub.p], the tool has no effect on the adjacent groove while forming the current groove. The fin heights thus exceed the corresponding ploughing-extrusion depths. Moreover, because the main forming angle is larger than the subsidiary one, the inclination of the wall of the previous groove is larger than that of its wall formed by the subsidiary forming surface. Then, with an increase in [a.sub.p], the current groove meets the deformation zone of the previous slot during the forming process. The cutter also begins to trim the last slot, resulting in a decline in the fin height. As seen from Fig. 4, c, when ap is 0.3 mm, the fin height only reaches 0.27 mm, therefore, the microgroove becomes narrower, increasing its depth-to-width ratio; fostering the accumulation of fluid and gas, facilitating the boiling of the working medium. When [a.sub.p] increases to 0.5 mm, the little cutting feed leads to large amounts of excess metal generation, resulting in the formation of extensive burn, hindering the forming process. The fin is only 0.43 mm high (smaller than [a.sub.p]) because of the substantial trimming by the cutting tool, as shown in Fig. 4, d.
Fig. 5 shows the changes in the proportion of the fin height to the ploughing-extrusion depth. When [Florin] or [Guilder][.sub.p] is kept constant, the ratio of the fin height to the ploughing-extrusion depth declines with [a.sub.p], despite an increase in the fin height. This increase can be explained by the intensifying trimming effect on the previous fin in this stage. On the other hand, when [a.sub.p] is kept unchanged, and [Florin] or [Guilder][.sub.p] increases, the ratio of the fin height to the ploughing-extrusion depth peaks before declining. When [Florin] or [Guilder][.sub.p] is small, a strong trimming effect reduces the fin height; as [Florin] or [Guilder][.sub.p] increases, the trimming effect is weakened and eventually disappears, thus increasing the fin height. Moreover, for different values of [a.sub.p], the peaks of the ratio appear at various values of [Florin] or [Guilder][.sub.p], ranging from 0.8 to 1.2 mm. Therefore, [a.sub.p] and [Florin] or [Guilder][.sub.p] have simultaneous impacts on the formation of microgrooves.
[FIGURE 5 OMITTED]
3.3. Effect of cutting feed on ploughing-extrusion speed
During the forming process of the boiling structure of microgrooves, the ploughing-extrusion speed of the cutting tool keeps changing. In the beginning, when the cutter works on the outer edge of the work piece, the ploughing-extrusion speed is the highest. As the cutter moves toward the center of the workpiece, the process gradually slows down. The following nonlinear equation can describe the relationship among [d.sub.b], [Florin] or [Guilder][.sub.p], and [n.sub.p]:
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1)
where [V.SUB.P-E] represents the ploughing-extrusion speed of the cutting tool, T is the processing time, and [n.sub.p] is the rotational speed.
[FIGURE 6 OMITTED]
[FIGURE 7 OMITTED]
The ploughing-extrusion speed [V.SUB.P-E] changes with [Florin] or [Guilder][.sub.p] and T. When [Florin] or [Guilder][.sub.p] remains unchanged, [V.SUB.P-E] decreases with time, and the forming time decreases with [Florin] or [Guilder][.sub.p], as shown in Fig. 6.
According to Fig. 7, for a regular [Florin] or [Guilder][.sub.p], the processing time decreases with increasing rotational speed. This decrease tends to be sharp when [Florin] or [Guilder][.sub.p] is relatively small. The processing time also shows a decline with [Florin] or [Guilder][.sub.p]. In the ploughing-extrusion process, [Florin] or [Guilder][.sub.p] and [a.sub.p] jointly determine the structure of the microgrooves: [Florin] or [Guilder][.sub.p] and [a.sub.p] affects the microgroove shape in an interactive manner. Considering that the critical value of cutting feed is [f.sub.pcv] and the critical value of ploughing-extrusion depth is [a.sub.pcv], the conditions for the formation of a structure that combines V-shaped and U-shaped grooves and Eqs. can describe a structure that consists of only V-shaped grooves. Eqs. (2) and (3):
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2)
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (3)
1. The structure composed of spiral circumferential grooves, realized by employing the ploughing-extrusion method, enhancing the boiling performance of a heat transfer surface. In the ploughing-extrusion process, the critical value of [Florin] or [Guilder][.sub.p] varies with [a.sub.p]. When [a.sub.p] is kept unchanged, the fin height peaks when [Florin] or [Guilder][.sub.p] reaches its critical value. For a regular [Florin] or [Guilder][.sub.p], when [a.sub.p] is below the corresponding critical value, the fin height exceeds [a.sub.p]. When [a.sub.p] is above the critical value, the fin height fall below [a.sub.p]. Once [a.sub.p] increases beyond the critical point, the ratio of the fin height to the ploughing-extrusion depth reduces.
2. The ratio of the fin height to the ploughing-extrusion depth is affected by [Florin] or [Guilder][.sub.p] and [a.sub.p] simultaneously. For a regular [Florin] or [Guilder][.sub.p], the ratio of the fin height to the ploughing-extrusion depth decreases with an increase in [a.sub.p], despite an increase in the fin height. When [a.sub.p] is kept unchanged, and [Florin] or [Guilder][.sub.p] increases, the ratio of the fin height to the ploughing-extrusion depth rises to its peak value and then reduces. Moreover, for different values of [a.sub.p], the peaks of the ratio appear at various values of [Florin] or [Guilder][.sub.p].
3. In the ploughing-extrusion process, [Florin] or [Guilder][.sub.p] and [a.sub.p] together determine the conditions for the formation of the groove structure, which can be a structure that combines V-shaped and U-shaped grooves or a structure that consists of only V-shaped grooves.
This research were supported by the National Nature Science Foundation of China (51575115, 51275099); Project (Kfkt2014-06) supported by Open Research Fund of Key Laboratory of High Performance Complex Manufacturing, Central South University; the project of Guangdong research program (Yq2013127, 2014A010105053); Project (201510010069) supported by Guangzhou research program; Guangzhou Key Laboratory for Monitoring and Control of Electromechanical Equipment (2060402).
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J.H. Xiang, J.A. Duan, C.L. Zhang, H.B. Zhou, F. Jian, C. Zhou
FORMATION MECHANISM FOR PLANAR STRUCTURE CONSISTING OF CIRCUMFERENTIAL SPIRAL GROOVES
The use of a boiling structure to enhance heat transfer can help improve evaporation efficiency and heat transfer capacity. In this paper, a planar boiling structure consisting of integrated spiral V-shaped grooves is proposed; the forming process of this structure, i.e., a ploughing-extrusion method based on a copper plate, is also presented. The relationships among the ploughing-extrusion speed [V.sub.P-E], rotational speed [n.sub.p], and cutting feed [Florin] or [Guilder][.sub.p] are discussed. The effect of the cutting feed on the fin height and the ratio of the fin height to the ploughing-extrusion depth is studied. The relationship among the ploughing-extrusion speed, cutting feed, and processing time and that among the processing time, cutting feed, and rotational speed is analyzed. The experiment also examines the effect of related parameters on the structure formation and their appropriate combinations, determining the conditions required to form a structure consisting of only a V-shaped groove.
Keywords: Spiral grooves, Ploughing-extrusion method, V-shaped grooves, Microstructure.
Received September 30, 2016
Accepted April 14, 2017
J.H. Xiang(*), J.A. Duan(**), C.L. Zhang(***), H.B. Zhou(****), F. Jian(*****), C. Zhou(******)
(*) School of Mechanical and Electric Engineering, Guangzhou University, Guangzhou 510006, China
(**) College of Mechanical and Electrical Engineering, Central South University, Changsha 410083, China
(***) School of Mechanical and Electric Engineering, Guangzhou University, Guangzhou 510006, China
(****) College of Mechanical and Electrical Engineering, Central South University, Changsha 410083, China
(*****) School of Mechanical and Electric Engineering, Guangzhou University, Guangzhou 510006, China
(******) School of Mechanical and Electric Engineering, Guangzhou University, Guangzhou 510006, China
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|Author:||Xiang, J.H.; Duan, J.A.; Zhang, C.L.; Zhou, H.B.; Jian, F.; Zhou, C.|
|Date:||Mar 1, 2017|
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