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Thermal performance of a boiling and condensation enhanced heat transfer tube—stepped lattice finned tube
Applied Thermal Engineering 173 (2020) 115227
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Thermal performance of a boiling and condensation enhanced heat transfer tube—stepped lattice finned tube
T
Hanping Chena, Haijun Moa, Zhenping Wana, , Shufeng Huangb, Xiaowu Wanga, Hongguan Zhuc ⁎
a
School of Mechanical and Automotive Engineering, South China University of Technology, Guangzhou 510640, PR China East China University of Technology, Nanchang 330032, PR China c Guangdong Longfeng Precision Copper Tube Co., Ltd, Zhuhai 519000, PR China b
HIGHLIGHTS
enhanced tube (SLFT) with two layers of staggered stepped fins is designed. • AThenovel enhances condensation heat transfer by 7.1 times compared with smooth tube. • The SLFT enhances boiling heat transfer by 4.1 times compared with smooth tube. • The SLFT • SLFT can be used to condensation as well as boiling heat transfer enhancement. ARTICLE INFO
ABSTRACT
Keywords: Heat transfer enhancement Condensation Boiling Finned tube
It is a challenging job to develop a finned tube that possesses highly efficient condensation as well as boiling heat transfer enhancement. A stepped lattice finned tube (SLFT) with two layers of staggered fins produced on the side wall of spiral grooves of tube surface is designed and fabricated to achieve the goal. The overall heat transfer performance, shell side and tube side heat transfer performance of the SLFT in condensation and boiling conditions are experimentally investigated at the flow velocity ranging from 1 m/s to 3 m/s. The overall condensation and boiling heat transfer coefficients of the SLFT are up to 8.1 times and 5.1 times that of the smooth tube, respectively. Compared with other reported enhanced tubes, the SLFT has better condensation and boiling heat transfer performance. The SLFT with moderate pitch, small step height and small width possesses better condensation heat transfer enhancement whereas the SLFT with large pitch, large step height and moderate width possesses better boiling heat transfer enhancement. The SLFT with superior heat transfer enhancement under both condensation and boiling conditions provides a promising solution to highly efficient and compact heat exchanger integrated refrigerating and heating.
1. Introduction The integration of efficient refrigerating and heating is demanded for shell and tube heat exchangers to meet the requirements of compact structure [1,2]. For instance, heat pump and central air-conditioning units not only need to achieve efficient refrigerating, but also efficient heating [3,4]. To achieve the integration of refrigerating and heating in a heat exchanger, the heat exchange tube is required to have both high condensation and boiling heat transfer efficiency [5–7]. However, the currently available enhanced heat exchange tubes can only be applied in a single condition because the enhanced heat transfer structures for boiling and condensation are distinctly different. The surface porous tube and finned tube are the main evaporation
⁎
tubes studied and applied [8–10]. The surface porous tube is a kind of efficient heat exchange tube with a large number of pores on the surface as the vaporization core to improve the boiling heat transfer performance. According to different processing methods, surface porous tubes are divided into sintered porous tube [11–13], flame sprayed porous tube [14], electroplated porous tube [15], mechanically processed porous tube [16], chemically corroded porous tube [17], laser-processed porous tube [18] and so on. Another kind of enhanced boiling heat transfer tube—finned tube, such as scaly finned tube and doubly enhanced tube [19–20], can make boiling heat transfer coefficient 3–5 times higher than smooth tube. However, the evaporation tubes mentioned above are only suitable for a single boiling heat transfer condition, and its condensation performance is far lower than that of the
Corresponding author. E-mail address: [email protected] (Z. Wan).
https://doi.org/10.1016/j.applthermaleng.2020.115227 Received 17 December 2019; Received in revised form 22 February 2020; Accepted 19 March 2020 Available online 21 March 2020 1359-4311/ © 2020 Elsevier Ltd. All rights reserved.
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condensation tubes when applied to condensation condition. The reason is that the surface porous tube and evaporation finned tube have good hydrophilicity, which will lead to the accumulation of condensate film in the condensation condition. Integral finned tube and anisotropic enhanced tube, which can increase the heat transfer area and thin the condensate film, are the two main enhanced condensation heat transfer tubes. The finned tubes with high condensation heat transfer coefficients mainly include C-shaped finned tube [21], serrated finned tube [22], petal finned tube [23], edge-shaped finned tube [24] and so on. The common anisotropic enhanced tubes include spiral groove tube [25,26], spiral flat tube [27] and corrugated tube [28], which can enhance the heat transfer performance inside and outside the tube. However, the anisotropic enhanced tubes are more difficult to be manufactured and result in larger pressure drop. What’s more, the condensation tubes mentioned above are only suitable for a single condensation heat transfer condition, and its boiling performance is much lower than that of the evaporation tubes when applied to the boiling condition because integral finned tube and anisotropic enhanced tube cannot provide a large number of boiling nucleation points. Therefore, the currently available enhanced heat exchange tubes cannot meet the requirements of modern heat exchanger integrated refrigerating and heating. Hence, in order to effectively improve the refrigerating and heating performance of the modern heat exchanger, such as heat pump and central air-conditioning units, it is urgent to develop an enhanced heat transfer tube with excellent heat transfer enhancement performance under both boiling and condensation conditions. This work is aiming to design a new three-dimensional finned tube which can not only greatly enhance the condensation heat transfer, but also the boiling heat transfer. The overall heat transfer performance, shell side and tube side heat transfer performance of the new threedimensional finned tube are experimentally investigated in condensation and boiling conditions. In addition, the influence of fin parameters on condensation and boiling heat transfer is investigated and the corresponding enhanced heat transfer mechanisms are discussed. Moreover, the comparison of heat transfer performance between the new three-dimensional finned tube and other reported enhanced tubes is conducted.
Fig. 1. Design process of the stepped lattice finned tube: (a) basic structure of SLFT; (b) toothed fins; (c) staggered and stepped lattice fins; (d) sectional view of SLFT.
promoting the condensate film thinning. Therefore, a layer of intermittent toothed fins should be added to the side wall of the spiral low fins, as shown in Fig. 1(b). At the same time, the intermittent toothed fins can effectively increase the heat transfer area and provide more boiling cores to a certain extent. But it is insufficient for enhancing boiling heat transfer. So, another staggered layer of intermittent toothed fins is added to the side walls to further enhance the boiling heat transfer, as shown in Fig. 1(c). The two layers of intermittent toothed fins located at different heights on the side walls interdigitate to form a porous structure. The slot of the spiral fins is divided into upper and lower parts. Moreover, the semi-closed lower fin slot forms a concave channel and is connected with the upper fin slot through clearance holes, which is expected to promote the flow of the working fluid. This kind of enhanced heat transfer tube with the above fin structure is called as stepped lattice finned tube (SLFT). In addition, there should be an enhanced structure inside the tube since the total thermal resistance is determined by the outside and inside structures of the heat exchange tube. Hence, as shown in Fig. 1(d), a trapezoidal threaded groove structure is employed inside the tube to achieve high heat transfer efficiency as well as a low pressure drop.
2. Design and characterization of enhanced structure 2.1. Enhanced structure for condensation and boiling heat transfer In general, the enhanced structure of condensation and boiling has contradictory requirements. The enhanced structure of a condensation tube is required to be hydrophobic and can promote thinning and detaching of the condensate film; while the enhanced structure of a boiling tube is required to have good wettability and can provide more nucleation sites and facilitate the departure of bubbles [29]. Therefore, the main problem in design of enhanced structure for condensation and boiling heat transfer is how to balance the dilemma to achieve high condensing efficiency as well as high boiling efficiency. Studies have shown that a low finned tube is simple but effective condensation tube [30,31], and spiral structure can reduce the accumulation of condensate film. On the other hand, high-density spiral low fin structure can promote the generation of boiling bubbles. Therefore, the dense spiral low fins as shown in Fig. 1(a) are used as the basic structure of the enhanced condensation and boiling heat transfer tube. In addition, the concave grooves can increase surface tension and promote the formation of bubbles. Hence, the dense spiral low fins should be combined with the concave groove, as shown in Fig. 1(a). However, the side walls of the spiral low fins are flat and smooth, which is disadvantageous for the spreading of the condensate film, thus resulting in condensate film accumulation at the root of fins. As we know, the intermittent tooth structure can destroy the continuity of condensate film, thereby
2.2. Characterization of the SLFT The main characteristic parameters of the SLFT include pitch (P) of the helical fin, step height (ΔH) and width (w) of the stepped fins, as shown in Fig. 1(c) and (d). The SLFT is successfully fabricated by an innovative self-developed rolling-plowing combination tool. The base tube for the SLFT is a copper tube having a length of 2500 mm, an outer diameter of 19 mm, and a thickness of 1.15 mm. The structure of the stepped lattice finned tube manufactured by a self-developed rolling-plowing combination tool is shown in Fig. 2. It can be seen from Fig. 2(b) and (c) that the two-layer stepped fins exhibit 2
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Fig. 2. The structure of the stepped lattice finned tube: (a) appearance photo; (b) and (c) staggered and stepped lattice fins; (d) inner fins.
a lattice-like distribution, resulting in slots between the helix fins being divided into upper and lower parts. Hence, the effective heat transfer area of the SLFT are greatly increased. The height of the helical fin is 1 mm and the pitch are in range of 0.61–0.71 mm, which is consistent with the design requirements of the dense spiral low fins. In particular, there are two layers of intermittently stepped fins of varying heights in the fin slot, as shown in Fig. 2(b). Two layers of stepped fins are distributed crisscrossing on the side wall of helical fins along the circumferential direction. And the stepped fins have sharp edges and a rough surface. In addition, the lower stepped fins are in contact with each other in the same fin slot, while the upper stepped fins are kept at a certain distance to form clearance holes, which makes the fins present a lattice-like distribution as a whole, as shown in Fig. 2(c). Under the separation of the stepped fins, the lower part of the fin slot becomes a semi-closed chamber and the holes formed between the stepped fins connect the upper and lower slots. The chambers communicate with each other to form a spiral channel, which enables to promote the flow of the liquid. And the bottom of the channel is concave, which enables to achieve the design goal of increasing surface tension. Both the channel and holes are key to enhancing boiling and condensation heat transfer processes. Fig. 2(d) shows the morphology of the inner fins which is a trapezoidal internal thread structure with a height of 0.35 mm. In general, the actual structure of the SLFT is highly consistent with the design goal, and the design idea is realized. The thermal performance of seven stepped lattice finned tubes with different parameters as shown in Table. 1 are tested in present work.
contains a refrigerant circuit, a heating circuit and a cooling circuit. The refrigerant circuit consisted of an evaporator and a condenser is the core of the test setup. The SLFT or smooth tube are installed in the evaporator and condenser as the heat exchange tube. The tubes to be tested have an effective length of 2500 mm so that the upstream section is extended enough to ensure a fully developed flow generated in the tube. A smooth tube and seven SLFTs with different structural parameters as shown in Table. 1 are tested one by one. The seven finned tubes are labeled FT-1, FT-2 to FT-7 while the smooth tube is labeled ST. 3.2. Test operation Before conducting the experiments, nitrogen with a pressure of 10 MPa is charged into the test system and hold for 24 h so as to ensure a good sealing condition. Then, the test system is pumped to a vacuum of 30 Pa and hold for 6 h. This step is to exclude impurity gases. Afterwards, the refrigerant, R134a, is charged into the evaporator until it fills two-thirds of the volume of the evaporator. After finishing the above operations, the test can be conducted. As shown in Fig. 3, the hot water provided by a hot water tank with a built-in heater is pumped to the heat exchange tubes of the evaporator. Then, the refrigerant is heated by the hot water and evaporates into vapor with a certain temperature and pressure. The vapor will be condensed by the cold water provided by the cooling circuit when it enters the shell side of the condenser. The condensate falls into the evaporator due to gravity for the next working cycle. Simultaneously, the hot water and the cold water flow back into the hot/cold water tank for the next cycle, respectively. In order to ensure the accuracy and stability of the test results, each tube is individually tested and one of the boiling and condensation heat transfer performance is tested at each time. During the condensation testing, the saturation temperature of R134a is maintained at 40 °C by controlling the temperature and flow velocity of the water, and the vapor pressure is approximately 1016 kPa in the condenser. The inlet temperature of the cold water is maintained at 32 °C. During the boiling testing, the saturation temperature of R134a is maintained at 6 °C by controlling the temperature and flow velocity of the water, and the vapor pressure is approximately 362 kPa in the evaporator. The inlet temperature of the hot water is maintained at 12 °C. The flow velocities are measured by rotameters with an accuracy of 0.5%. The pressures of condenser and evaporator are measured using pressure transmitter with an accuracy of 1% while a differential pressure transmitter with an accuracy of 0.5% is used to measure the pressure drop inside the finned tube. After calibrated, the K-type thermocouples with an accuracy of 0.1 °C are used to measure
3. Experimental procedures 3.1. Test setup Fig. 3 schematically illustrates the test setup for boiling and condensation performance of the SLFT. The experimental setup mainly Table 1 Structure parameters of stepped lattice finned tube in present study. Tube
P (mm)
ΔH (mm)
W (mm)
SLFT-1 SLFT-2 SLFT-3 SLFT-4 SLFT-5 SLFT-6 SLFT-7
0.66 0.66 0.66 0.66 0.66 0.61 0.71
0.10 0.10 0.10 0.15 0.20 0.15 0.15
0.33 0.30 0.27 0.27 0.27 0.27 0.27
3
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1-computer rotameter
2-data acquisition system 3-hot water tank 4-water pump 5-control valve
7-differential pressure transmitter
6-
8-evaporator 9-condenser 10-finned tube 11-cold
water tank 12-filter Fig. 3. Schematic diagram of evaporation and condensation heat transfer performance. Experimental setup.
the inlet and outlet temperatures of the hot and cold water and the vapor temperatures in condenser and evaporator. When the saturation temperature of the vapor fluctuates within 0.1 °C and lasts at least 10 min, the experimental data are collected every 30 s and ten groups of data are collected. All the temperature and pressure data are transmitted to a data acquisition system and processed by a computer. Each experiment is repeated three times and the arithmetic averages are adopted as the final result. To reduce heat loss, each part of the test system is thermally insulated using insulation cotton.
are also in a countercurrent state. The heat taken away by the cold water in the condenser is calculated by:
Qc = qvc c cp,c (Ti,c
where qvc is the volume flow velocity of cold water, Ti,c and To,c are the inlet and outlet temperatures of the cold water, and h and cp,c are the density and specific heat of the cold water, respectively. So the overall heat transfer coefficient under condensation condition is calculated as:
Uc =
3.3. Data analysis The aim of this section is to obtain the overall heat transfer coefficients U of the finned tube. The method and process are given as follows.
To,h )
Qh Ao Tmh
q=
(1)
Ti,h Tsat
Ti,h
Q Ao
h i = STCi
(8)
(9)
k 0.8 1/3 µ Re Pr di µw
0.14
(10)
where STCi coefficient should be obtained by Wilson plot technique. The uncertainty of the overall heat transfer coefficient can be defined as:
To,h To,h
(7)
Since the SLFT is a double-enhanced tube, the tube side heat transfer coefficient (hi) should be calculated by the following formula.
(3)
Tsat
Ti,c To,c
1 1 Ao = + + Rw U ho Ai h i
(2)
Ao = do L
ln
Ti,c
T
The relationship between the overall heat transfer coefficient and the heat transfer coefficient inside and outside the tube is as follow:
where Tm is the logarithmic mean temperature, Ao is the nominal heat transfer area of the tube.
Tmh =
To,c ln Tsat
The heat flux is calculated as follows:
where qv h is the volume flow velocity of hot water, Ti,h and To,h are the inlet and outlet temperatures of the hot water, h and c p,h are the density and specific heat of the hot water, respectively. So the overall heat transfer coefficient under pool boiling condition is calculated as:
Ub =
(6)
sat
The hot water in the tube side and the refrigerant in the shell side are in a countercurrent state. The heat absorbed from the hot water in the evaporator is calculated by: h c p,h (Ti,h
Qc A o Tmc
Tmc =
(1) Boiling condition:
Q h = qv h
(5)
To,c )
U = U
(4)
(2) Condensation condition:
Q Q
2
+
A A
2
+
Tm Tm
2 1/2
(11)
The measurement error of L and d is 1 mm. Hence, the maximum uncertainty of the boiling and condensation heat transfer coefficient in this work are 6.3% and 5.4%, respectively.
The cold water in the tube side and the refrigerant in the shell side 4
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3.4. Verification of experimental setup
transfer coefficients (Uc) and overall boiling heat transfer coefficients (Ub) rise as the water flow velocity inside the SLTF increases. Furthermore, the overall heat transfer coefficients of the SLFTs under condensation and boiling conditions are much larger than that of the smooth tube, regardless of the P, ΔH and w. It indicates that the SLFT can greatly enhance both condensation and boiling heat transfer and break through the limitation of traditional enhanced tube. From Fig. 5(a), the heat transfer coefficients and variation trends of the SLFTs with a pitch of 0.61 mm and 0.71 mm are very close, while the pitch of 0.66 mm is the optimum condition for condensation heat transfer. This is because a too large pitch reduces the effective heat transfer area, while a too small pitch will inhibit the flow and separation of condensate. The overall heat transfer coefficients of SLFTs with pitch of 0.61 mm, 0.66 mm and 0.71 mm are 4.9–6.5 times, 5.2–7.0 times and 4.7–6.3 times that of smooth tube in the flow velocity range of 1.0–3.0 m/s under condensation condition. Under boiling condition, the SLFT with P = 0.66 mm exhibits better performance when the water flow velocity is less than 1.5 m/s, while the SLFT with P = 0.71 mm performs better when the water flow velocity is larger than 1.5 m/s, as shown in Fig. 6(a). And with the increase of water flow velocity, the overall boiling heat transfer performance of the SLFT with a pitch of 0.71 mm is improved more significant than that with a pitch of 0.61 mm and 0.66 mm. The unique trend comes from the variation law of the shell side condensation heat transfer coefficients with pitch. The overall boiling heat transfer coefficient of SLFT is up to 10665 W/ m2·K which is 5.1 times that of smooth tube. From Fig. 5(b), it can be seen that the overall heat transfer coefficient of the SLFT increases as the step height decreases under condensation condition. When ΔH = 0.10 and V = 3.0 m/s, the Uc reaches 13149 W/m2·K that is 8.1 times that of smooth tube. On the contrary, the overall boiling heat transfer coefficient of the SLFT decreases as the step height decreases under boiling condition as shown in Fig. 6(b). From Fig. 5(c), the Uc decreases monotonically with the increase of the fin width. Also, the Uc of the three SLFTs maintains an excellent level regardless of the fin width and is 7.1–8.1 times that of the smooth tube at a flow velocity of 3 m/s. Differently, the Ub of the SLFT with a fin width of 0.30 mm is better than that of the other two SLFTs with width of 0.33 mm and 0.27 mm as shown in Fig. 6(c). The results show that the Ub of the SLFT with w = 0.30 mm is 3.7–4.3 times that of the smooth tube. Comparatively, the Ub of the SLFT with w = 0.33 mm and 0.27 mm are only 3.2–3.6 times and 3.3–4.2 times that of the smooth tube, respectively. Therefore, the SLFT with a pitch of 0.66 mm, a step height of 0.10 mm and a width of 0.27 mm possesses better overall condensation heat transfer enhancement, whereas the SLFT with a pitch of 0.71 mm, a step height of 0.20 mm and a width of 0.30 mm exhibits better boiling heat transfer enhancement.
In this work, the legitimacy of the experimental setup is verified by comparing the heat transfer coefficients and pressure drop obtained from the experiments of the smooth tube with the results obtained from the standard correlations. Since the flow state of the condensate film of the saturation refrigerant vapor outside the smooth tube is laminar, the condensation heat transfer coefficient (hoc ) on the single smooth tube can be calculated according to Eq. (12).
hoc = 0.725
2 l g
µl d o (Tsat
1 4
3 l
Tw )
(12)
where l is the liquid density of the refrigerant, g is the gravitational acceleration, is the refrigerant latent heat of vaporization, I »l and µl are the liquid thermal conductivity and viscosity of the refrigerant, Tw is the wall temperature. The boiling heat transfer coefficient of the horizontal tube hob is calculated as Eq. (13).
hob = 90q0.67M
m = 0.12
0.5P m ( R
lgPR )
0.55
(13) (14)
0.2lgRp
where M is the molecular weight of the refrigerant, PR is the ratio of working fluid pressure to critical pressure and Rp is the surface roughness of the tube. The Nusselt number in the tube is calculated according to the equation of Dittus-Boelter as Eq. (15). (15)
Nuth = 0.023Re0.8 Pr n
Comparisons of condensation and boiling heat transfer coefficients obtained from experiments outside the smooth tube with the theoretical calculation results are shown in Fig. 4, respectively. The results show that the data obtained from the validation experiments are in good agreement with those obtained from the correlations, and the deviation are within 3.0% and 5.5% for the condensation and boiling heat transfer coefficients, respectively. Hence, the experimental results are guaranteed. 4. Results and discussion 4.1. Overall thermal performance of the SLFT Figs. 5 and 6 demonstrates the variation of the overall condensation and boiling heat transfer coefficients of SLFTs with varied pitch (P), step height (ΔH) and width (w) at different water flow velocities (V) inside the tube. From Figs. 5 and 6, both the overall condensation heat
Fig. 4. Comparisons of heat transfer coefficients outside the smooth tube with the correlations. 5
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Fig. 5. Variation of overall condensation heat transfer coefficients versus water flow velocity V for SLFT with different structural parameters.
Therefore, the SLFT has excellent heat transfer enhancement under both condensation and boiling conditions. The reason can be attributed to the follow: (i) The channel formed by the lower slots of the SLFT provides a four-sided heated environment for the working medium, which enables the working medium to be heated continuously to form bubbles and quickly to escape. And the clearance holes formed by the stepped fins provide a large number of nucleation sites and create channels between the upper and lower fin slots, thus facilitating the escape of bubbles and the influx of ambient cryogenic liquid into the channel. (ii) The holes connecting the lower and upper fin slots can generate capillary force to promote the condensate flow, while the stepped fins effectively thin the condensate film and promote it separating from the tube. (iii) The rough surface of the stepped fins can provide more active nucleation sites for boiling heat transfer, while the sharp edges of the fins can break condensate during condensation. (iv) The trapezoidal thread fins inside the tube expand the heat transfer area and destroy the fluid boundary layer, which can enhance convective heat transfer inside the tube. (v) The stepped lattice fins increase the shell side heat transfer area greatly. Furthermore, moderate pitch, small step height and small width are more conducive to the flow and thinning of condensate film, whereas small pitch and fin width are not conducive to the separation of boiling bubbles. In addition, small step height and large fin width may reduce the number of boiling nucleate points. Therefore, the SLFT with moderate pitch, step height and width can balance the dilemma in optimum between condensation and boiling heat transfer performance.
condensation and boiling heat transfer coefficients of SLFTs with different pitch (P), step height (ΔH) and width (w) at different heat flux (q). It can be found from Fig. 7 that the shell side condensation heat transfer coefficients (hoc) increase slightly or remain stable as the heat flux increases, which is inconsistent with the characteristic of the traditional condensation heat transfer tubes. The shell side heat transfer coefficients of traditional condensation heat transfer tubes decrease with the increase of the heat flux because of the accumulation of condensate. This indicates that the SLFT can effectively prevent condensate accumulating as the heat flux increases. It can be attributed to the stepped fins, clearance holes and concave channel. Two layers of staggered stepped fins effectively increase surface tension and destroy the continuity of the condensate film, which promotes the spread of the condensate. And the helical concave channel provides passage to guide the condensate flow and exert a centrifugal force to produce secondary flow. As a result, the thickness of the condensate film reduces rapidly and efficiently. In addition, the clearance holes connecting the upper and lower fin slots can facilitate the flow of condensate from the upper slot to the lower slot. Therefore, the condensate can be quickly detached from the tube, thus avoiding the accumulation of condensate. Similar to the trends shown in Fig. 5(a), the SLFT with a pitch of 0.66 mm presents a better hoc than the SLFTs with pitches of 0.61 mm and 0.71 mm as shown in Fig. 7(a), which indicates that the pitch of 0.66 mm is more conducive to the flow and detachment of condensate. From Fig. 8(a), the shell side boiling heat transfer coefficients (hob) of the SLFTs with different pitches present varied trends with the increase of heat flux. The hob of the SLFT with a pitch of 0.71 mm increases monotonically as the heat flux increases, while the SLFT with a pitch of 0.61 mm and 0.66 mm is reversed. Specially, the hob of SLFT with a pitch of 0.61 mm and 0.66 mm achieves a peak value at a heat
4.2. Shell side thermal performance of the SLFT Figs. 7 and 8 demonstrates the variation of the shell side 6
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Fig. 6. Variation of overall boiling heat transfer coefficients versus water flow velocity V for SLFT with different structural parameters.
flux of 27,000–30,000 W/m2. It is indicated that the SLFTs with pitches of 0.61 mm and 0.66 mm reach the heat flux limit earlier because the small pitch is unfavorable for the detachment of the bubbles, thus resulting in deterioration of the boiling heat transfer process. And the SLFT with a pitch of 0.71 mm, having a larger heat flux limit, performs better under high heat flux conditions. It can be seen from Figs. 7(b) and 8(b) that the SLFT with a step height of 0.10 mm presents the best shell side heat transfer coefficient during condensation, but exhibits the worst shell side heat transfer coefficient during boiling. This phenomenon is due to the fact that the requirements of enhanced structure for condensation and boiling heat transfer are distinctly different. The reason is that the holes formed by stepped fins are getting small when the step height decrease, thus resulting in larger capillary force to promote the flow of condensate. However, stepped fins with larger step height have lots of rough corners and dimples, which can provide larger amount of the nucleate points. Furthermore, as the step height decrease, the escape of the boiling bubbles will be restrained. Therefore, a large step height is more conducive to promoting boiling heat transfer. In addition, as shown in Fig. 7(b), the hoc decreases with the increasing of heat flux when ΔH = 0.10 mm, and the hoc increases slightly with the increasing of heat flux when ΔH is 0.15 mm and 0.20 mm. That is because the condensate completely submerges the stepped fins when the heat flux is large so that the influence of the capillary force is weakened. However, at this time, the enhancement of the stepped fins on the disturbance of the condensate becomes significant. Hence, the hoc of the SLFT with large step height increases slightly with the increase of heat flux. From
Fig. 8(b), it can be seen that with the increase of heat flux, the hob of SLFTs with different step heights has completely different trend. And the boiling heat flux limits are all around 30,000 W/m2 because the pitches are all 0.66 mm, which is consistent with the result of the Fig. 8(a). As shown in Fig. 7(c), the SLFT with small w achieves a better hoc because of the larger surface tension and capillary force resulted by a small w. And the hoc keeps stable as the heat flux increases in the range of 30,000–70,000 W/m2. However, the SLFT with a fin width of 0.30 mm presents the best hob, which can be seen from Fig. 8(c). The reasons are as follows: (i) The number of stepped lattice fins per unit area decrease as the w increases, which result in a decrease in the number of nucleate points. Also, the heat transfer area decreases with the increase of w. Thus, the hob of SLFT with a fin width of 0.33 mm is smaller than that of SLFT with a fin width of 0.30 mm. (ii) The SLFT with a fin width of 0.27 mm has small clearance holes, which restricts the escape of bubbles. As a result, the SLFT with a fin width of 0.27 mm presents unsatisfied heat transfer performance under boiling condition, especially under high heat flux. Hence, in order to get an excellent overall heat transfer performance, a moderate fin width should be chosen. 4.3. Thermo-hydraulic characteristic of tube side It can be seen from Fig. 9 that the tube-side heat transfer coefficient of the SLFTs is much larger than that of the smooth tube under both condensation and boiling conditions. The tube side condensation heat
7
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Fig. 7. Variation of shell side condensation heat transfer coefficients versus heat flux q for SLFT with different structural parameters.
transfer coefficient (hic) and tube side boiling heat transfer coefficient (hib) of the SLFTs are about 3.2 times and 2.6 times that of the smooth tube, respectively. The reason is that the thread grooves of SLFTs increase heat transfer area and promote turbulence inside the tube. The hic of the SLFT-1 to SLFT-7 is basically the same and so does the hib, because they have the same internal thread structure parameters. With the increase of the water flow velocity, the enhancement inside the tube is more pronounced. On the other hand, the pressure drop inside the SLFTs is 1.5–1.9 times that of the smooth tube, seen from Fig. 10. The reason is that the thread grooves increase the contact area between fluid and tube wall and promote turbulence intensity. Hence, the flow resistance of the fluid inside the SLFT increases, leading to an increase of pressure drop. Nevertheless, it can be seen from Fig. 11 that the PEC (performance evaluation criterion) values inside the SLFTs are respectively 2.48–2.79 and 2.11–2.36 times that of the smooth tube under condensation and boiling condition. Hence, the stepped lattice fins and internal thread structure enable the finned tube to achieve synergistic heat transfer enhancement.
condensation heat transfer coefficient and the shell side condensation heat transfer coefficient of the SLFT are larger than that of edge-shaped finned tube [24], petal-shaped finned tube [23], integral-fin tube [32], enhanced tube having 3D roughness [33] and 3D finned tube [34]. Compared with above condensation tube, the average shell-side condensation heat transfer enhancement factor of SLFT increase by 27.7%, 84.0%, 82.1%, 91.6% and 13.5%, respectively. The reason is that the staggered stepped fins of the SLFT can effectively promote the flow and thinning of condensate. Compared with doubly enhanced tube [35], twisted tube with machined porous surface [36], enhanced tube sintered with open-celled copper foam [12] and evaporation finned tube [37], the average shell-side boiling heat transfer enhancement factor of SLFT increase by 34%, 328%, 328% and 87%, respectively. Because the stepped lattice fins provide a lot of boiling nucleate points and the lower fin slots formed by the separated stepped fins effectively promotes the generation and escape of bubbles, thereby promoting boiling heat transfer. Although the shell-side boiling heat transfer performance of the SLFT is 8% lower than that of the sintered porous surface tube, the overall boiling heat transfer performance of the SLFT is 2.5 times than that of the sintered porous surface tube. Because SLFT has internal thread fin which can effectively enhance the tube side heat transfer. In conclusion, the SLFT having excellent condensation and boiling heat transfer performance, break through the limitation that traditional enhanced tubes can only be used to a single condensation or boiling working condition.
4.4. Comparison with previous work To achieve a more in-depth understanding of the condensation and boiling heat transfer enhancement of the SLFT, a comparison of the SLFT and several other enhanced tubes is conducted and results are shown in Tables 2 and 3. In a certain range of heat flux, the overall
8
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Fig. 8. Variation of shell side boiling heat transfer coefficients versus heat flux q for SLFT with different structural parameters.
5. Conclusions
SLFT are up to 8.1 times and 5.1 times that of the smooth tube, respectively. (2) The SLFT with moderate pitch, small step height and small width possesses better condensation heat transfer performance, whereas the SLFT with large pitch, large step height and moderate width possesses better boiling heat transfer performance. Comprehensively, the SLFT with a pitch of 0.66 mm, a step height
(1) The SLFT designed possesses excellent enhanced condensation heat transfer as well as enhanced boiling heat transfer and can break through the limitation that traditional condensation or boiling enhanced tubes are only suitable for single working condition. The overall condensation and boiling heat transfer coefficients of the
Fig. 9. Tube side heat transfer coefficient versus water flow velocity V for stepped lattice finned tube.
9
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Fig. 10. Pressure drop inside the tube versus water flow velocity V for stepped lattice finned tube.
Fig. 11. PEC inside the tube versus water flow velocity V for stepped lattice finned tube. Table 2 Comparison of enhanced condensation heat transfer between the SLFT and other heat transfer tube.
Table 3 Comparison of boiling heat transfer performance between the SLFT and other heat transfer tube.
References
Configuration
Uc/Ust
hoc/hst
q (kW/m2)
References
Configuration
Ub/Ust
hob/hst
q (kW/m2)
Present study Wan [24] Qin [23] Kumar [32] Kim [33]
SLFT Edge-shaped finned tube Petal-shaped finned tube Integral-fin tube Enhanced tube having 3D roughness 3D finned tube
4.7–8.1 2.7–3.4 \ \ \
6.3–12.1 4.7–9.7 4.6–5.4 4.2–5.9 3.3–6.3
30–72 30–100 25–75 20–60 30–70
Present study Yang [35] Gao [36]
3.2–5.1 3.2–3.6 \
3.5–12.8 3.3–8.8 1.8–2.0
17–47 15–45 10–60
\
6.5–9.7
30–85
SLFT Doubly enhanced tube Twisted tube with machine process porous surface [18] Doubly enhanced tube Enhanced tube sintered with open-celled copper foam Evaporation Finned tube Sintered porous surface tube
\
2.2–2.7 1.4–2.4
10–60
1.8–2.0 2.4–2.6
4.2–4.5 7.9–9.8
18–50 18–50
Peng [34]
Zhang [20] Ji [12] Yang [37] Yang [37]
of 0.15 mm and a width of 0.30 mm possess the optimum balance between condensation and boiling heat transfer performance. (3) The reason that the SLFT can not only enhance condensation heat transfer, but also boiling heat transfer is that the stepped lattice fins can promote condensate flowing and separating from the tube, and at the same time, provide large amount of nucleate points and promote the generation and escape of boiling bubbles. In addition, the trapezoidal thread inner fin can effectively enhance the convective heat transfer in the tube. (4) Compared with other reported enhanced tubes, the average shell-
side condensation and boiling heat transfer enhancement factor of the SLFT increased by 13.5–91.6% and 34–328%, respectively. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. 10
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Acknowledgements
[18] R.C. Chu, K.P. Moran, Method for customizing nucleate boiling heat transfer from electronic units immersed in dielectric coolant, U.S. Patent (1997) 4050507. [19] D.C. Zhang, S.N. Tian, A.L. Zhao, X.W. Fan, Experimental investigation on boiling heat transfer of R22 outside three doubly-enhanced tubes, Int. Conf. Mater. Renewable Energy Environ. 2 (2011) 1897–1901. [20] D.C. Zhang, K. Wang, Y.L. He, W.Q. Tao, Boiling heat transfer of R134a outside horizontal doubly-enhanced tubes, J. Chem. Ind. Eng. (China) 58 (2007) 2710. [21] D.C. Zhang, W.T. Ji, W.Q. Tao, Condensation heat transfer of HFC134a on horizontal low thermal conductivity tubes, Int. Commun. Heat Mass Transfer 34 (8) (2007) 917–923. [22] A. Lemouedda, A. Schmid, E. Franz, M. Breuer, A. Delgado, Numerical investigations for the optimization of serrated finned-tube heat exchangers, Appl. Therm. Eng. 31 (2011) 1393–1401. [23] P. Qin, Z.Y. Ling, T. Xu, Z.G. Zhang, Z.C. Ye, Comparison on condensation heat transfer performance of R407C outside petal-shaped finned tube with low finned tube, J. Eng. Thermo Phys. 34 (12) (2013) 2347–2350. [24] Z.P. Wan, X.W. Wang, Y. Tang, Heat transfer performance of an edge-shaped finned tube, Heat Transfer Eng. 36 (6) (2015) 574–581. [25] B.B. Wang, X.Q. Qiu, Research status and developing trend of heat transfer technology, Ind. Heating 35 (4) (2006) 48–51. [26] J. Li, M.L.J.Y. Yu, J. Liu, S.T. Ji Du, Numerical simulation for thermal characteristic of three different structure heat exchange tubes, Acta Energiae Solaris Sinica 33 (5) (2012) 839–845. [27] S. Koyama, A. Miyara, H. Takamatsu, T. Fujii, Condensation heat transfer of binary refrigerant mixtures of R-22 and R-114 inside a horizontal tube with internal spiral grooves, Int. J. Refrig. 13 (4) (1990) 256–263. [28] S. Laohalertdecha, S. Wongwises, Condensation heat transfer and flow characteristics of R-134a flowing through corrugated tube, Int. J. Heat Mass Transf. 54 (2011) 2673–2682. [29] T. Ebisu, Evaporation and condensation heat transfer enhancement for alternative refrigerants used in air-conditioning machines, Heat Transfer Enhance. Heat Exchangers (1999) 579–600. [30] H.S. Wang, J.W. Rose, Effect of interphase matter transfer on condensation on lowfinned tubes––a theoretical investigation, Int. J. Heat Mass Transf. 47 (1) (2004) 179–184. [31] A. Reif, A. Büchner, S. Rehfeldt, H. Klein, Outer heat transfer coefficient for condensation of pure components on single horizontal low-finned tubes, Heat Mass Transfer 55 (1) (2017) 3–16. [32] R. Kumar, A. Gupta, S. Vishvakarma, Condensation of R-134a vapour over single horizontal integral-fin tubes: effect of fin height, Int. J. Refrig. 28 (3) (2005) 428–435. [33] N.H. Kim, Condensation of R-134a on horizontal enhanced tubes having three-dimensional roughness, Int. J. Air-Condit. Refrig. 24 (02) (2016) 1650013. [34] P. Qin, Z.G. Zhang, T. Xu, X.N. Gao, S.F. Wang, Experimental investigation on condensation heat transfer of R134a on single horizontal copper and stainless steel three-dimensional finned tubes, Am. Inst. Phys. 1547 (2013) 513–521. [35] G.Y. Yang, S.H. Zhang, X.L. Li, Experimental study on boiling heat transfer of R134a on single horizontal doubly-enhanced tubes, J. Refrig. 28 (1) (2007) 26–31. [36] X.N. Gao, H.B. Yin, Y.Y. Huang, S.M. Ling, Z.G. Zhang, Y.T. Fang, Heat transfer of boiling R134a and R142b on a twisted tube with machine processed porous surface, Chin. J. Chem. Eng. 16 (3) (2008) 492–496. [37] J. Yang, The Evaporation Characteristic of High Efficient Heat Transfer Tube in Refrigeration System, East China University of Science and Technology, China, 2015.
This work is sponsored by the Key Research and Development Project of Guangdong Province (2019B090914001) and Guangdong University Students Science and Technology Innovation Cultivation Project (pdjh2019b0032). References [1] S.F. Huang, Z.P. Wan, Y. Tang, Manufacturing and single-phase thermal performance of an arc-shaped inner finned tube for heat exchanger, Appl. Therm. Eng. 158 (2019) 1–8. [2] S.F. Huang, H.G. Zhu, Z.P. Wan, Compound thermal performance of an arc-shaped inner finned tube equipped with Y-branch inserts, Appl. Therm. Eng. 152 (2019) 475–481. [3] Q.L. Wang, S.F. Sun, L. Wang, Experimental research on a dual heat source air conditioner with water heater in heating mode in winter, Cryogenics Supercond. 35 (2007) 262–265. [4] Y.X. Gu, T. Wang, W. Liu, F.F. Ma, Energy-saving analysis and heat transfer performance of wastewater source heat pump, Appl. Mech. Mater. 694 (2014) 211–217. [5] Y.H. Zhang, D.G. Lu, Z.Y. Wang, X.L. Fu, Q. Cao, Y.H. Yang, Experimental investigation on pool-boiling of C-shape heat exchanger bundle used in PRHR HX, Appl. Therm. Eng. 114 (2016) 186–195. [6] D.J. Kukulka, R. Smith, Thermal-hydraulic performance of Vipertex 1EHT enhanced heat transfer tubes, Appl. Therm. Eng. 61 (1) (2013) 60–66. [7] H.P. Chen, Z.P. Wan, H.J. Mo, S.F. Huang, Y. Tang, Experimental studies on the compound thermo-hydraulic characteristics in a 3D inner finned tube with porous copper fiber inserts, Int. Commun. Heat Mass Transfer 100 (2019) 51–59. [8] J.L. Liu, Y.L. Dai, X.M. Xia, Manufacture and application of high efficiency boiling tube for heat exchanger, Adv. Mater. Res. 236 (2011) 1640–1644. [9] H. Xu, Y.L. Dai, H.H. Cao, J.L. Liu, L. Zhang, Tubes with coated and sintered porous surface for highly efficient heat exchangers, Front. Chem. Sci. Eng. 12 (3) (2018) 367–375. [10] Y. Li, C.Q. Yan, L.C. Sun, Enhancement of pool boiling heat transfer on porous surface tube, Nucl. Power Eng. 32 (2) (2011) 63–67. [11] S.Z. Zhou, T.B. Hou, M.Q. Pan, Y. Han, Y. Chen, Production of sintered porous tube and the study of boiling heat transfer, Equip. Environ. Eng. 11 (3) (2014) 91–95. [12] W.T. Ji, Z.G. Qu, Z.Y. Li, J.F. Guo, D.C. Zhang, W.Q. Tao, Pool boiling heat transfer of R134a on single horizontal tube surfaces sintered with open-celled copper foam, Int. J. Therm. Sci. 50 (11) (2011) 2248–2255. [13] X.S. Wang, Z.B. Wang, Q.Z. Chen, Research on manufacturing technology and heat transfer characteristics of sintered porous surface tubes, Adv. Mater. Res. 97 (2010) 1161–1165. [14] Y. Zeng, H. Xu, F. Hou, Y.L. Dai, J.L. Liu, Property studies on the flame-sprayed type surface porous tubes, Chem. Eng. Machinery 37 (2010) 141–144. [15] C.E. Albertson, Boiling heat transfer surface and method, U.S. Patent, 1997, 4018264. [16] G. Jiang, H.G. Ma, Experimental study on evaporation outside the surface enhanced tubes, J. Univ. Shanghai Sci. Technol. 30 (5) (2008) 465–469. [17] H. Pu, G.L. Ding, H.T. Hu, Y.F. Gao, Effect of salt spray corrosion on air-side hydrophilicity and thermal-hydraulic performance of copper-fin heat exchangers, Heat Mass Transf. 46 (8–9) (2010) 859–867.
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Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Thermal performance of a boiling and condensation enhanced heat transfer tube—stepped lattice finned tube
T
Hanping Chena, Haijun Moa, Zhenping Wana, , Shufeng Huangb, Xiaowu Wanga, Hongguan Zhuc ⁎
a
School of Mechanical and Automotive Engineering, South China University of Technology, Guangzhou 510640, PR China East China University of Technology, Nanchang 330032, PR China c Guangdong Longfeng Precision Copper Tube Co., Ltd, Zhuhai 519000, PR China b
HIGHLIGHTS
enhanced tube (SLFT) with two layers of staggered stepped fins is designed. • AThenovel enhances condensation heat transfer by 7.1 times compared with smooth tube. • The SLFT enhances boiling heat transfer by 4.1 times compared with smooth tube. • The SLFT • SLFT can be used to condensation as well as boiling heat transfer enhancement. ARTICLE INFO
ABSTRACT
Keywords: Heat transfer enhancement Condensation Boiling Finned tube
It is a challenging job to develop a finned tube that possesses highly efficient condensation as well as boiling heat transfer enhancement. A stepped lattice finned tube (SLFT) with two layers of staggered fins produced on the side wall of spiral grooves of tube surface is designed and fabricated to achieve the goal. The overall heat transfer performance, shell side and tube side heat transfer performance of the SLFT in condensation and boiling conditions are experimentally investigated at the flow velocity ranging from 1 m/s to 3 m/s. The overall condensation and boiling heat transfer coefficients of the SLFT are up to 8.1 times and 5.1 times that of the smooth tube, respectively. Compared with other reported enhanced tubes, the SLFT has better condensation and boiling heat transfer performance. The SLFT with moderate pitch, small step height and small width possesses better condensation heat transfer enhancement whereas the SLFT with large pitch, large step height and moderate width possesses better boiling heat transfer enhancement. The SLFT with superior heat transfer enhancement under both condensation and boiling conditions provides a promising solution to highly efficient and compact heat exchanger integrated refrigerating and heating.
1. Introduction The integration of efficient refrigerating and heating is demanded for shell and tube heat exchangers to meet the requirements of compact structure [1,2]. For instance, heat pump and central air-conditioning units not only need to achieve efficient refrigerating, but also efficient heating [3,4]. To achieve the integration of refrigerating and heating in a heat exchanger, the heat exchange tube is required to have both high condensation and boiling heat transfer efficiency [5–7]. However, the currently available enhanced heat exchange tubes can only be applied in a single condition because the enhanced heat transfer structures for boiling and condensation are distinctly different. The surface porous tube and finned tube are the main evaporation
⁎
tubes studied and applied [8–10]. The surface porous tube is a kind of efficient heat exchange tube with a large number of pores on the surface as the vaporization core to improve the boiling heat transfer performance. According to different processing methods, surface porous tubes are divided into sintered porous tube [11–13], flame sprayed porous tube [14], electroplated porous tube [15], mechanically processed porous tube [16], chemically corroded porous tube [17], laser-processed porous tube [18] and so on. Another kind of enhanced boiling heat transfer tube—finned tube, such as scaly finned tube and doubly enhanced tube [19–20], can make boiling heat transfer coefficient 3–5 times higher than smooth tube. However, the evaporation tubes mentioned above are only suitable for a single boiling heat transfer condition, and its condensation performance is far lower than that of the
Corresponding author. E-mail address: [email protected] (Z. Wan).
https://doi.org/10.1016/j.applthermaleng.2020.115227 Received 17 December 2019; Received in revised form 22 February 2020; Accepted 19 March 2020 Available online 21 March 2020 1359-4311/ © 2020 Elsevier Ltd. All rights reserved.
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condensation tubes when applied to condensation condition. The reason is that the surface porous tube and evaporation finned tube have good hydrophilicity, which will lead to the accumulation of condensate film in the condensation condition. Integral finned tube and anisotropic enhanced tube, which can increase the heat transfer area and thin the condensate film, are the two main enhanced condensation heat transfer tubes. The finned tubes with high condensation heat transfer coefficients mainly include C-shaped finned tube [21], serrated finned tube [22], petal finned tube [23], edge-shaped finned tube [24] and so on. The common anisotropic enhanced tubes include spiral groove tube [25,26], spiral flat tube [27] and corrugated tube [28], which can enhance the heat transfer performance inside and outside the tube. However, the anisotropic enhanced tubes are more difficult to be manufactured and result in larger pressure drop. What’s more, the condensation tubes mentioned above are only suitable for a single condensation heat transfer condition, and its boiling performance is much lower than that of the evaporation tubes when applied to the boiling condition because integral finned tube and anisotropic enhanced tube cannot provide a large number of boiling nucleation points. Therefore, the currently available enhanced heat exchange tubes cannot meet the requirements of modern heat exchanger integrated refrigerating and heating. Hence, in order to effectively improve the refrigerating and heating performance of the modern heat exchanger, such as heat pump and central air-conditioning units, it is urgent to develop an enhanced heat transfer tube with excellent heat transfer enhancement performance under both boiling and condensation conditions. This work is aiming to design a new three-dimensional finned tube which can not only greatly enhance the condensation heat transfer, but also the boiling heat transfer. The overall heat transfer performance, shell side and tube side heat transfer performance of the new threedimensional finned tube are experimentally investigated in condensation and boiling conditions. In addition, the influence of fin parameters on condensation and boiling heat transfer is investigated and the corresponding enhanced heat transfer mechanisms are discussed. Moreover, the comparison of heat transfer performance between the new three-dimensional finned tube and other reported enhanced tubes is conducted.
Fig. 1. Design process of the stepped lattice finned tube: (a) basic structure of SLFT; (b) toothed fins; (c) staggered and stepped lattice fins; (d) sectional view of SLFT.
promoting the condensate film thinning. Therefore, a layer of intermittent toothed fins should be added to the side wall of the spiral low fins, as shown in Fig. 1(b). At the same time, the intermittent toothed fins can effectively increase the heat transfer area and provide more boiling cores to a certain extent. But it is insufficient for enhancing boiling heat transfer. So, another staggered layer of intermittent toothed fins is added to the side walls to further enhance the boiling heat transfer, as shown in Fig. 1(c). The two layers of intermittent toothed fins located at different heights on the side walls interdigitate to form a porous structure. The slot of the spiral fins is divided into upper and lower parts. Moreover, the semi-closed lower fin slot forms a concave channel and is connected with the upper fin slot through clearance holes, which is expected to promote the flow of the working fluid. This kind of enhanced heat transfer tube with the above fin structure is called as stepped lattice finned tube (SLFT). In addition, there should be an enhanced structure inside the tube since the total thermal resistance is determined by the outside and inside structures of the heat exchange tube. Hence, as shown in Fig. 1(d), a trapezoidal threaded groove structure is employed inside the tube to achieve high heat transfer efficiency as well as a low pressure drop.
2. Design and characterization of enhanced structure 2.1. Enhanced structure for condensation and boiling heat transfer In general, the enhanced structure of condensation and boiling has contradictory requirements. The enhanced structure of a condensation tube is required to be hydrophobic and can promote thinning and detaching of the condensate film; while the enhanced structure of a boiling tube is required to have good wettability and can provide more nucleation sites and facilitate the departure of bubbles [29]. Therefore, the main problem in design of enhanced structure for condensation and boiling heat transfer is how to balance the dilemma to achieve high condensing efficiency as well as high boiling efficiency. Studies have shown that a low finned tube is simple but effective condensation tube [30,31], and spiral structure can reduce the accumulation of condensate film. On the other hand, high-density spiral low fin structure can promote the generation of boiling bubbles. Therefore, the dense spiral low fins as shown in Fig. 1(a) are used as the basic structure of the enhanced condensation and boiling heat transfer tube. In addition, the concave grooves can increase surface tension and promote the formation of bubbles. Hence, the dense spiral low fins should be combined with the concave groove, as shown in Fig. 1(a). However, the side walls of the spiral low fins are flat and smooth, which is disadvantageous for the spreading of the condensate film, thus resulting in condensate film accumulation at the root of fins. As we know, the intermittent tooth structure can destroy the continuity of condensate film, thereby
2.2. Characterization of the SLFT The main characteristic parameters of the SLFT include pitch (P) of the helical fin, step height (ΔH) and width (w) of the stepped fins, as shown in Fig. 1(c) and (d). The SLFT is successfully fabricated by an innovative self-developed rolling-plowing combination tool. The base tube for the SLFT is a copper tube having a length of 2500 mm, an outer diameter of 19 mm, and a thickness of 1.15 mm. The structure of the stepped lattice finned tube manufactured by a self-developed rolling-plowing combination tool is shown in Fig. 2. It can be seen from Fig. 2(b) and (c) that the two-layer stepped fins exhibit 2
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Fig. 2. The structure of the stepped lattice finned tube: (a) appearance photo; (b) and (c) staggered and stepped lattice fins; (d) inner fins.
a lattice-like distribution, resulting in slots between the helix fins being divided into upper and lower parts. Hence, the effective heat transfer area of the SLFT are greatly increased. The height of the helical fin is 1 mm and the pitch are in range of 0.61–0.71 mm, which is consistent with the design requirements of the dense spiral low fins. In particular, there are two layers of intermittently stepped fins of varying heights in the fin slot, as shown in Fig. 2(b). Two layers of stepped fins are distributed crisscrossing on the side wall of helical fins along the circumferential direction. And the stepped fins have sharp edges and a rough surface. In addition, the lower stepped fins are in contact with each other in the same fin slot, while the upper stepped fins are kept at a certain distance to form clearance holes, which makes the fins present a lattice-like distribution as a whole, as shown in Fig. 2(c). Under the separation of the stepped fins, the lower part of the fin slot becomes a semi-closed chamber and the holes formed between the stepped fins connect the upper and lower slots. The chambers communicate with each other to form a spiral channel, which enables to promote the flow of the liquid. And the bottom of the channel is concave, which enables to achieve the design goal of increasing surface tension. Both the channel and holes are key to enhancing boiling and condensation heat transfer processes. Fig. 2(d) shows the morphology of the inner fins which is a trapezoidal internal thread structure with a height of 0.35 mm. In general, the actual structure of the SLFT is highly consistent with the design goal, and the design idea is realized. The thermal performance of seven stepped lattice finned tubes with different parameters as shown in Table. 1 are tested in present work.
contains a refrigerant circuit, a heating circuit and a cooling circuit. The refrigerant circuit consisted of an evaporator and a condenser is the core of the test setup. The SLFT or smooth tube are installed in the evaporator and condenser as the heat exchange tube. The tubes to be tested have an effective length of 2500 mm so that the upstream section is extended enough to ensure a fully developed flow generated in the tube. A smooth tube and seven SLFTs with different structural parameters as shown in Table. 1 are tested one by one. The seven finned tubes are labeled FT-1, FT-2 to FT-7 while the smooth tube is labeled ST. 3.2. Test operation Before conducting the experiments, nitrogen with a pressure of 10 MPa is charged into the test system and hold for 24 h so as to ensure a good sealing condition. Then, the test system is pumped to a vacuum of 30 Pa and hold for 6 h. This step is to exclude impurity gases. Afterwards, the refrigerant, R134a, is charged into the evaporator until it fills two-thirds of the volume of the evaporator. After finishing the above operations, the test can be conducted. As shown in Fig. 3, the hot water provided by a hot water tank with a built-in heater is pumped to the heat exchange tubes of the evaporator. Then, the refrigerant is heated by the hot water and evaporates into vapor with a certain temperature and pressure. The vapor will be condensed by the cold water provided by the cooling circuit when it enters the shell side of the condenser. The condensate falls into the evaporator due to gravity for the next working cycle. Simultaneously, the hot water and the cold water flow back into the hot/cold water tank for the next cycle, respectively. In order to ensure the accuracy and stability of the test results, each tube is individually tested and one of the boiling and condensation heat transfer performance is tested at each time. During the condensation testing, the saturation temperature of R134a is maintained at 40 °C by controlling the temperature and flow velocity of the water, and the vapor pressure is approximately 1016 kPa in the condenser. The inlet temperature of the cold water is maintained at 32 °C. During the boiling testing, the saturation temperature of R134a is maintained at 6 °C by controlling the temperature and flow velocity of the water, and the vapor pressure is approximately 362 kPa in the evaporator. The inlet temperature of the hot water is maintained at 12 °C. The flow velocities are measured by rotameters with an accuracy of 0.5%. The pressures of condenser and evaporator are measured using pressure transmitter with an accuracy of 1% while a differential pressure transmitter with an accuracy of 0.5% is used to measure the pressure drop inside the finned tube. After calibrated, the K-type thermocouples with an accuracy of 0.1 °C are used to measure
3. Experimental procedures 3.1. Test setup Fig. 3 schematically illustrates the test setup for boiling and condensation performance of the SLFT. The experimental setup mainly Table 1 Structure parameters of stepped lattice finned tube in present study. Tube
P (mm)
ΔH (mm)
W (mm)
SLFT-1 SLFT-2 SLFT-3 SLFT-4 SLFT-5 SLFT-6 SLFT-7
0.66 0.66 0.66 0.66 0.66 0.61 0.71
0.10 0.10 0.10 0.15 0.20 0.15 0.15
0.33 0.30 0.27 0.27 0.27 0.27 0.27
3
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1-computer rotameter
2-data acquisition system 3-hot water tank 4-water pump 5-control valve
7-differential pressure transmitter
6-
8-evaporator 9-condenser 10-finned tube 11-cold
water tank 12-filter Fig. 3. Schematic diagram of evaporation and condensation heat transfer performance. Experimental setup.
the inlet and outlet temperatures of the hot and cold water and the vapor temperatures in condenser and evaporator. When the saturation temperature of the vapor fluctuates within 0.1 °C and lasts at least 10 min, the experimental data are collected every 30 s and ten groups of data are collected. All the temperature and pressure data are transmitted to a data acquisition system and processed by a computer. Each experiment is repeated three times and the arithmetic averages are adopted as the final result. To reduce heat loss, each part of the test system is thermally insulated using insulation cotton.
are also in a countercurrent state. The heat taken away by the cold water in the condenser is calculated by:
Qc = qvc c cp,c (Ti,c
where qvc is the volume flow velocity of cold water, Ti,c and To,c are the inlet and outlet temperatures of the cold water, and h and cp,c are the density and specific heat of the cold water, respectively. So the overall heat transfer coefficient under condensation condition is calculated as:
Uc =
3.3. Data analysis The aim of this section is to obtain the overall heat transfer coefficients U of the finned tube. The method and process are given as follows.
To,h )
Qh Ao Tmh
q=
(1)
Ti,h Tsat
Ti,h
Q Ao
h i = STCi
(8)
(9)
k 0.8 1/3 µ Re Pr di µw
0.14
(10)
where STCi coefficient should be obtained by Wilson plot technique. The uncertainty of the overall heat transfer coefficient can be defined as:
To,h To,h
(7)
Since the SLFT is a double-enhanced tube, the tube side heat transfer coefficient (hi) should be calculated by the following formula.
(3)
Tsat
Ti,c To,c
1 1 Ao = + + Rw U ho Ai h i
(2)
Ao = do L
ln
Ti,c
T
The relationship between the overall heat transfer coefficient and the heat transfer coefficient inside and outside the tube is as follow:
where Tm is the logarithmic mean temperature, Ao is the nominal heat transfer area of the tube.
Tmh =
To,c ln Tsat
The heat flux is calculated as follows:
where qv h is the volume flow velocity of hot water, Ti,h and To,h are the inlet and outlet temperatures of the hot water, h and c p,h are the density and specific heat of the hot water, respectively. So the overall heat transfer coefficient under pool boiling condition is calculated as:
Ub =
(6)
sat
The hot water in the tube side and the refrigerant in the shell side are in a countercurrent state. The heat absorbed from the hot water in the evaporator is calculated by: h c p,h (Ti,h
Qc A o Tmc
Tmc =
(1) Boiling condition:
Q h = qv h
(5)
To,c )
U = U
(4)
(2) Condensation condition:
Q Q
2
+
A A
2
+
Tm Tm
2 1/2
(11)
The measurement error of L and d is 1 mm. Hence, the maximum uncertainty of the boiling and condensation heat transfer coefficient in this work are 6.3% and 5.4%, respectively.
The cold water in the tube side and the refrigerant in the shell side 4
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3.4. Verification of experimental setup
transfer coefficients (Uc) and overall boiling heat transfer coefficients (Ub) rise as the water flow velocity inside the SLTF increases. Furthermore, the overall heat transfer coefficients of the SLFTs under condensation and boiling conditions are much larger than that of the smooth tube, regardless of the P, ΔH and w. It indicates that the SLFT can greatly enhance both condensation and boiling heat transfer and break through the limitation of traditional enhanced tube. From Fig. 5(a), the heat transfer coefficients and variation trends of the SLFTs with a pitch of 0.61 mm and 0.71 mm are very close, while the pitch of 0.66 mm is the optimum condition for condensation heat transfer. This is because a too large pitch reduces the effective heat transfer area, while a too small pitch will inhibit the flow and separation of condensate. The overall heat transfer coefficients of SLFTs with pitch of 0.61 mm, 0.66 mm and 0.71 mm are 4.9–6.5 times, 5.2–7.0 times and 4.7–6.3 times that of smooth tube in the flow velocity range of 1.0–3.0 m/s under condensation condition. Under boiling condition, the SLFT with P = 0.66 mm exhibits better performance when the water flow velocity is less than 1.5 m/s, while the SLFT with P = 0.71 mm performs better when the water flow velocity is larger than 1.5 m/s, as shown in Fig. 6(a). And with the increase of water flow velocity, the overall boiling heat transfer performance of the SLFT with a pitch of 0.71 mm is improved more significant than that with a pitch of 0.61 mm and 0.66 mm. The unique trend comes from the variation law of the shell side condensation heat transfer coefficients with pitch. The overall boiling heat transfer coefficient of SLFT is up to 10665 W/ m2·K which is 5.1 times that of smooth tube. From Fig. 5(b), it can be seen that the overall heat transfer coefficient of the SLFT increases as the step height decreases under condensation condition. When ΔH = 0.10 and V = 3.0 m/s, the Uc reaches 13149 W/m2·K that is 8.1 times that of smooth tube. On the contrary, the overall boiling heat transfer coefficient of the SLFT decreases as the step height decreases under boiling condition as shown in Fig. 6(b). From Fig. 5(c), the Uc decreases monotonically with the increase of the fin width. Also, the Uc of the three SLFTs maintains an excellent level regardless of the fin width and is 7.1–8.1 times that of the smooth tube at a flow velocity of 3 m/s. Differently, the Ub of the SLFT with a fin width of 0.30 mm is better than that of the other two SLFTs with width of 0.33 mm and 0.27 mm as shown in Fig. 6(c). The results show that the Ub of the SLFT with w = 0.30 mm is 3.7–4.3 times that of the smooth tube. Comparatively, the Ub of the SLFT with w = 0.33 mm and 0.27 mm are only 3.2–3.6 times and 3.3–4.2 times that of the smooth tube, respectively. Therefore, the SLFT with a pitch of 0.66 mm, a step height of 0.10 mm and a width of 0.27 mm possesses better overall condensation heat transfer enhancement, whereas the SLFT with a pitch of 0.71 mm, a step height of 0.20 mm and a width of 0.30 mm exhibits better boiling heat transfer enhancement.
In this work, the legitimacy of the experimental setup is verified by comparing the heat transfer coefficients and pressure drop obtained from the experiments of the smooth tube with the results obtained from the standard correlations. Since the flow state of the condensate film of the saturation refrigerant vapor outside the smooth tube is laminar, the condensation heat transfer coefficient (hoc ) on the single smooth tube can be calculated according to Eq. (12).
hoc = 0.725
2 l g
µl d o (Tsat
1 4
3 l
Tw )
(12)
where l is the liquid density of the refrigerant, g is the gravitational acceleration, is the refrigerant latent heat of vaporization, I »l and µl are the liquid thermal conductivity and viscosity of the refrigerant, Tw is the wall temperature. The boiling heat transfer coefficient of the horizontal tube hob is calculated as Eq. (13).
hob = 90q0.67M
m = 0.12
0.5P m ( R
lgPR )
0.55
(13) (14)
0.2lgRp
where M is the molecular weight of the refrigerant, PR is the ratio of working fluid pressure to critical pressure and Rp is the surface roughness of the tube. The Nusselt number in the tube is calculated according to the equation of Dittus-Boelter as Eq. (15). (15)
Nuth = 0.023Re0.8 Pr n
Comparisons of condensation and boiling heat transfer coefficients obtained from experiments outside the smooth tube with the theoretical calculation results are shown in Fig. 4, respectively. The results show that the data obtained from the validation experiments are in good agreement with those obtained from the correlations, and the deviation are within 3.0% and 5.5% for the condensation and boiling heat transfer coefficients, respectively. Hence, the experimental results are guaranteed. 4. Results and discussion 4.1. Overall thermal performance of the SLFT Figs. 5 and 6 demonstrates the variation of the overall condensation and boiling heat transfer coefficients of SLFTs with varied pitch (P), step height (ΔH) and width (w) at different water flow velocities (V) inside the tube. From Figs. 5 and 6, both the overall condensation heat
Fig. 4. Comparisons of heat transfer coefficients outside the smooth tube with the correlations. 5
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Fig. 5. Variation of overall condensation heat transfer coefficients versus water flow velocity V for SLFT with different structural parameters.
Therefore, the SLFT has excellent heat transfer enhancement under both condensation and boiling conditions. The reason can be attributed to the follow: (i) The channel formed by the lower slots of the SLFT provides a four-sided heated environment for the working medium, which enables the working medium to be heated continuously to form bubbles and quickly to escape. And the clearance holes formed by the stepped fins provide a large number of nucleation sites and create channels between the upper and lower fin slots, thus facilitating the escape of bubbles and the influx of ambient cryogenic liquid into the channel. (ii) The holes connecting the lower and upper fin slots can generate capillary force to promote the condensate flow, while the stepped fins effectively thin the condensate film and promote it separating from the tube. (iii) The rough surface of the stepped fins can provide more active nucleation sites for boiling heat transfer, while the sharp edges of the fins can break condensate during condensation. (iv) The trapezoidal thread fins inside the tube expand the heat transfer area and destroy the fluid boundary layer, which can enhance convective heat transfer inside the tube. (v) The stepped lattice fins increase the shell side heat transfer area greatly. Furthermore, moderate pitch, small step height and small width are more conducive to the flow and thinning of condensate film, whereas small pitch and fin width are not conducive to the separation of boiling bubbles. In addition, small step height and large fin width may reduce the number of boiling nucleate points. Therefore, the SLFT with moderate pitch, step height and width can balance the dilemma in optimum between condensation and boiling heat transfer performance.
condensation and boiling heat transfer coefficients of SLFTs with different pitch (P), step height (ΔH) and width (w) at different heat flux (q). It can be found from Fig. 7 that the shell side condensation heat transfer coefficients (hoc) increase slightly or remain stable as the heat flux increases, which is inconsistent with the characteristic of the traditional condensation heat transfer tubes. The shell side heat transfer coefficients of traditional condensation heat transfer tubes decrease with the increase of the heat flux because of the accumulation of condensate. This indicates that the SLFT can effectively prevent condensate accumulating as the heat flux increases. It can be attributed to the stepped fins, clearance holes and concave channel. Two layers of staggered stepped fins effectively increase surface tension and destroy the continuity of the condensate film, which promotes the spread of the condensate. And the helical concave channel provides passage to guide the condensate flow and exert a centrifugal force to produce secondary flow. As a result, the thickness of the condensate film reduces rapidly and efficiently. In addition, the clearance holes connecting the upper and lower fin slots can facilitate the flow of condensate from the upper slot to the lower slot. Therefore, the condensate can be quickly detached from the tube, thus avoiding the accumulation of condensate. Similar to the trends shown in Fig. 5(a), the SLFT with a pitch of 0.66 mm presents a better hoc than the SLFTs with pitches of 0.61 mm and 0.71 mm as shown in Fig. 7(a), which indicates that the pitch of 0.66 mm is more conducive to the flow and detachment of condensate. From Fig. 8(a), the shell side boiling heat transfer coefficients (hob) of the SLFTs with different pitches present varied trends with the increase of heat flux. The hob of the SLFT with a pitch of 0.71 mm increases monotonically as the heat flux increases, while the SLFT with a pitch of 0.61 mm and 0.66 mm is reversed. Specially, the hob of SLFT with a pitch of 0.61 mm and 0.66 mm achieves a peak value at a heat
4.2. Shell side thermal performance of the SLFT Figs. 7 and 8 demonstrates the variation of the shell side 6
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Fig. 6. Variation of overall boiling heat transfer coefficients versus water flow velocity V for SLFT with different structural parameters.
flux of 27,000–30,000 W/m2. It is indicated that the SLFTs with pitches of 0.61 mm and 0.66 mm reach the heat flux limit earlier because the small pitch is unfavorable for the detachment of the bubbles, thus resulting in deterioration of the boiling heat transfer process. And the SLFT with a pitch of 0.71 mm, having a larger heat flux limit, performs better under high heat flux conditions. It can be seen from Figs. 7(b) and 8(b) that the SLFT with a step height of 0.10 mm presents the best shell side heat transfer coefficient during condensation, but exhibits the worst shell side heat transfer coefficient during boiling. This phenomenon is due to the fact that the requirements of enhanced structure for condensation and boiling heat transfer are distinctly different. The reason is that the holes formed by stepped fins are getting small when the step height decrease, thus resulting in larger capillary force to promote the flow of condensate. However, stepped fins with larger step height have lots of rough corners and dimples, which can provide larger amount of the nucleate points. Furthermore, as the step height decrease, the escape of the boiling bubbles will be restrained. Therefore, a large step height is more conducive to promoting boiling heat transfer. In addition, as shown in Fig. 7(b), the hoc decreases with the increasing of heat flux when ΔH = 0.10 mm, and the hoc increases slightly with the increasing of heat flux when ΔH is 0.15 mm and 0.20 mm. That is because the condensate completely submerges the stepped fins when the heat flux is large so that the influence of the capillary force is weakened. However, at this time, the enhancement of the stepped fins on the disturbance of the condensate becomes significant. Hence, the hoc of the SLFT with large step height increases slightly with the increase of heat flux. From
Fig. 8(b), it can be seen that with the increase of heat flux, the hob of SLFTs with different step heights has completely different trend. And the boiling heat flux limits are all around 30,000 W/m2 because the pitches are all 0.66 mm, which is consistent with the result of the Fig. 8(a). As shown in Fig. 7(c), the SLFT with small w achieves a better hoc because of the larger surface tension and capillary force resulted by a small w. And the hoc keeps stable as the heat flux increases in the range of 30,000–70,000 W/m2. However, the SLFT with a fin width of 0.30 mm presents the best hob, which can be seen from Fig. 8(c). The reasons are as follows: (i) The number of stepped lattice fins per unit area decrease as the w increases, which result in a decrease in the number of nucleate points. Also, the heat transfer area decreases with the increase of w. Thus, the hob of SLFT with a fin width of 0.33 mm is smaller than that of SLFT with a fin width of 0.30 mm. (ii) The SLFT with a fin width of 0.27 mm has small clearance holes, which restricts the escape of bubbles. As a result, the SLFT with a fin width of 0.27 mm presents unsatisfied heat transfer performance under boiling condition, especially under high heat flux. Hence, in order to get an excellent overall heat transfer performance, a moderate fin width should be chosen. 4.3. Thermo-hydraulic characteristic of tube side It can be seen from Fig. 9 that the tube-side heat transfer coefficient of the SLFTs is much larger than that of the smooth tube under both condensation and boiling conditions. The tube side condensation heat
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Fig. 7. Variation of shell side condensation heat transfer coefficients versus heat flux q for SLFT with different structural parameters.
transfer coefficient (hic) and tube side boiling heat transfer coefficient (hib) of the SLFTs are about 3.2 times and 2.6 times that of the smooth tube, respectively. The reason is that the thread grooves of SLFTs increase heat transfer area and promote turbulence inside the tube. The hic of the SLFT-1 to SLFT-7 is basically the same and so does the hib, because they have the same internal thread structure parameters. With the increase of the water flow velocity, the enhancement inside the tube is more pronounced. On the other hand, the pressure drop inside the SLFTs is 1.5–1.9 times that of the smooth tube, seen from Fig. 10. The reason is that the thread grooves increase the contact area between fluid and tube wall and promote turbulence intensity. Hence, the flow resistance of the fluid inside the SLFT increases, leading to an increase of pressure drop. Nevertheless, it can be seen from Fig. 11 that the PEC (performance evaluation criterion) values inside the SLFTs are respectively 2.48–2.79 and 2.11–2.36 times that of the smooth tube under condensation and boiling condition. Hence, the stepped lattice fins and internal thread structure enable the finned tube to achieve synergistic heat transfer enhancement.
condensation heat transfer coefficient and the shell side condensation heat transfer coefficient of the SLFT are larger than that of edge-shaped finned tube [24], petal-shaped finned tube [23], integral-fin tube [32], enhanced tube having 3D roughness [33] and 3D finned tube [34]. Compared with above condensation tube, the average shell-side condensation heat transfer enhancement factor of SLFT increase by 27.7%, 84.0%, 82.1%, 91.6% and 13.5%, respectively. The reason is that the staggered stepped fins of the SLFT can effectively promote the flow and thinning of condensate. Compared with doubly enhanced tube [35], twisted tube with machined porous surface [36], enhanced tube sintered with open-celled copper foam [12] and evaporation finned tube [37], the average shell-side boiling heat transfer enhancement factor of SLFT increase by 34%, 328%, 328% and 87%, respectively. Because the stepped lattice fins provide a lot of boiling nucleate points and the lower fin slots formed by the separated stepped fins effectively promotes the generation and escape of bubbles, thereby promoting boiling heat transfer. Although the shell-side boiling heat transfer performance of the SLFT is 8% lower than that of the sintered porous surface tube, the overall boiling heat transfer performance of the SLFT is 2.5 times than that of the sintered porous surface tube. Because SLFT has internal thread fin which can effectively enhance the tube side heat transfer. In conclusion, the SLFT having excellent condensation and boiling heat transfer performance, break through the limitation that traditional enhanced tubes can only be used to a single condensation or boiling working condition.
4.4. Comparison with previous work To achieve a more in-depth understanding of the condensation and boiling heat transfer enhancement of the SLFT, a comparison of the SLFT and several other enhanced tubes is conducted and results are shown in Tables 2 and 3. In a certain range of heat flux, the overall
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Fig. 8. Variation of shell side boiling heat transfer coefficients versus heat flux q for SLFT with different structural parameters.
5. Conclusions
SLFT are up to 8.1 times and 5.1 times that of the smooth tube, respectively. (2) The SLFT with moderate pitch, small step height and small width possesses better condensation heat transfer performance, whereas the SLFT with large pitch, large step height and moderate width possesses better boiling heat transfer performance. Comprehensively, the SLFT with a pitch of 0.66 mm, a step height
(1) The SLFT designed possesses excellent enhanced condensation heat transfer as well as enhanced boiling heat transfer and can break through the limitation that traditional condensation or boiling enhanced tubes are only suitable for single working condition. The overall condensation and boiling heat transfer coefficients of the
Fig. 9. Tube side heat transfer coefficient versus water flow velocity V for stepped lattice finned tube.
9
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Fig. 10. Pressure drop inside the tube versus water flow velocity V for stepped lattice finned tube.
Fig. 11. PEC inside the tube versus water flow velocity V for stepped lattice finned tube. Table 2 Comparison of enhanced condensation heat transfer between the SLFT and other heat transfer tube.
Table 3 Comparison of boiling heat transfer performance between the SLFT and other heat transfer tube.
References
Configuration
Uc/Ust
hoc/hst
q (kW/m2)
References
Configuration
Ub/Ust
hob/hst
q (kW/m2)
Present study Wan [24] Qin [23] Kumar [32] Kim [33]
SLFT Edge-shaped finned tube Petal-shaped finned tube Integral-fin tube Enhanced tube having 3D roughness 3D finned tube
4.7–8.1 2.7–3.4 \ \ \
6.3–12.1 4.7–9.7 4.6–5.4 4.2–5.9 3.3–6.3
30–72 30–100 25–75 20–60 30–70
Present study Yang [35] Gao [36]
3.2–5.1 3.2–3.6 \
3.5–12.8 3.3–8.8 1.8–2.0
17–47 15–45 10–60
\
6.5–9.7
30–85
SLFT Doubly enhanced tube Twisted tube with machine process porous surface [18] Doubly enhanced tube Enhanced tube sintered with open-celled copper foam Evaporation Finned tube Sintered porous surface tube
\
2.2–2.7 1.4–2.4
10–60
1.8–2.0 2.4–2.6
4.2–4.5 7.9–9.8
18–50 18–50
Peng [34]
Zhang [20] Ji [12] Yang [37] Yang [37]
of 0.15 mm and a width of 0.30 mm possess the optimum balance between condensation and boiling heat transfer performance. (3) The reason that the SLFT can not only enhance condensation heat transfer, but also boiling heat transfer is that the stepped lattice fins can promote condensate flowing and separating from the tube, and at the same time, provide large amount of nucleate points and promote the generation and escape of boiling bubbles. In addition, the trapezoidal thread inner fin can effectively enhance the convective heat transfer in the tube. (4) Compared with other reported enhanced tubes, the average shell-
side condensation and boiling heat transfer enhancement factor of the SLFT increased by 13.5–91.6% and 34–328%, respectively. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. 10
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Acknowledgements
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