- Email: [email protected]
Experimental investigation of header configuration on two-phase flow distribution in plate-fin heat exchanger
International Communications in Heat and Mass Transfer 37 (2010) 116–120
Contents lists available at ScienceDirect
International Communications in Heat and Mass Transfer j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / i c h m t
Experimental investigation of header configuration on two-phase flow distribution in plate-fin heat exchanger☆ Simin Wang, Yanzhong Li, Jian Wen ⁎, Yansong Ma Department of Refrigeration and Cryogenics Engineering, School of Energy and Power Engineering, Xi'an Jiaotong University, Xi'an, 710049, China
a r t i c l e
i n f o
Available online 3 September 2009 Keyword: Plate-fin heat exchanger Two-phase flow Distribution Perforated baffle
a b s t r a c t The two-phase flow distribution in a plate-fin heat exchanger has been experimentally studied under different operation conditions. The results indicate that two-phase flow distribution is more complex and nonuniform than that of single-phase flow. The distribution uniformity of liquid-phase deteriorates with the decrease of Regas and Reliq. The distribution uniformity of gas-phase deteriorates with Reliq, but improves with Regas. The improved header with perforated baffle can effectively improve the uniformity of two-phase flow distribution and dryness distribution. The values of Sliq, Sgas and Sdry decrease by 5.4–44.0%, 4.7–35.0% and 11.7–30.0%, respectively. The conclusion is of great significance in the optimum design of plate-fin heat exchanger. © 2009 Elsevier Ltd. All rights reserved.
1. Introduction The use of plate-fin heat exchangers in industries around the world has increased considerably in recent years. They are widely used in process industries such as gas processing and petrochemical industries to exchange heat among more than two fluids with different supply temperatures due to their higher efficiency, more compact structure and lower costs than conventional two stream heat exchanger networks [1]. The common idealization in the basic plate-fin heat exchanger design theory is that the fluid is distributed uniformly at the inlet of the exchanger on each fluid channel throughout the core. However, in practice, flow maldistribution is more common and significantly reduces the idealized heat exchanger performance [2–4]. Heat exchangers in general and plate-fin heat exchangers in particular undergo decrease in performance due to flow maldistribution. The fluid tends to go preferentially into the channels that face the inlet tube because the inlet tube of plate-fin heat exchanger is small compared with the global size of the inlet header [5]. Much research has been done on the effects of flow nonuniformity on heat exchangers performance decrease. Based on the experimental data obtained from wind tunnel experiments, Chiou [6,7] set up a continuous flow distribution model and studied the thermal performance decrease in crossflow exchangers. Lalot et al. [8] presented the effect of flow nonuniformity on the performance of heat exchangers, based on the study of flow maldistribution in an experimental electrical heater. The results indicated that the flow maldistribution leads to a loss of effectiveness of about 25% for crossflow exchangers.
☆ Communicated by P. Cheng and W.Q. Tao. ⁎ Corresponding author. E-mail address: [email protected] (J. Wen). 0735-1933/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.icheatmasstransfer.2009.07.014
Ranganayakulu and Seetharamu [9] investigated the combined effects of wall longitudinal heat conduction, inlet flow uniformity and temperature nonuniformity on the thermal performance of a twofluid crossflow plate-fin heat exchanger using finite element method. The results showed that the performance may be reduced by 30% under extra nonuniform operating condition. For two-phase flow heat exchangers, the effect of flow maldistribution on performance is more serious, especially for the compact heat exchanger used in cryogenic plants. Since plate-fin heat exchangers are commonly used for small temperature difference, the slight maldistribution will result in serious decrease on heat transfer performance. It may even influence the mechanical behaviour of the heat exchangers in some cases [10]. Muller-Menzel et al. [11] observed various flow patterns in a plate-fin heat exchanger using a flow visualization rig. They found that at high gas mass fluxes both phases flow uniformly upwards. When decreasing the gas flow, a reversing slug flow becomes more and more pronounced. Nobuyuki Takenaka et al. [12] used real-time thermal neutron radiography to visualize boiling phenomena in a pool and boiling two-phase flow in an aluminum plate-fin heat exchanger. They all only investigated the phase distribution qualitatively inside only a single conduit because of the experimental limitation. The results of Muller and Chou [13] showed that the variation of dryness may result in the variation of pressure drop for two-phase flow, which will cause the maldistribution of two-phase flow in heat exchanger core. They also found that the unstable density-wave will cause surge of liquid-phase in heat exchanger. According to the previous studies, the effect of flow maldistribution on the thermal performance of a two-fluid crossflow heat exchanger is detrimental and not neglected. The use of a perforated baffle in the header can effectively enhance the distribution uniformity and heat transfer performance for single-phase flow. When air was adopted as
S. Wang et al. / International Communications in Heat and Mass Transfer 37 (2010) 116–120
117
Nomenclature N Re S w θ
the quantity of small zones Reynolds number maldistribution parameter measured value of flow rate or dryness ratio of the maximum value to the minimum
Subscripts ave average value dry dryness gas gas-phase i serial number of passage liq liquid-phase max maximum value min minimum value
Fig. 2. The division of outlet cross-section of heat exchanger.
the working fluid, the pressure drop of the improved heat exchanger increased by 277–1048 Pa, the heat transfer coefficient increases by 9.7–10.5% compared with the conventional one. The j and f factors increase by 8.0–11.5% and 15.6–45.6%, respectively [14]. No systematic study has been carried out for two-phase flow distribution. The objective of this paper is to experimentally study the two-phase flow distribution in plate-fin heat exchanger. The outlet distribution of both gas-phase and liquid-phase under different operation conditions was measured and the effects of inlet Reynolds number and header configuration on two-phase flow distribution were also investigated.
2 . Experimental system and evaluation of maldistribution 2.1. System description The schematic figure of experimental system is shown in Fig. 1. It is a multifunctional experimental system in which the experiments of both single-phase and two-phase flow distribution can be performed successfully. The experimental system consists of air loop, water loop and data acquisition system. Air is supplied by a compressor and is adjusted to required flow rate by different valves. The flow rate is measured by gas-turbine flowmeter. Water is pumped from tank to experimental tubes and its flow rate is measured by liquid-turbine flowmeter. Air and water are mixed evenly in the blender and then flow to the test part. The two-phase fluid at the outlet of heat exchanger flows into a switch box and separator to be separated into liquid and gas. The separated gas is measured by gas-turbine flowmeter and then is vented. The separated water flows downward into containers to be measured using weighing method. The measurement errors of the liquid velocity at the inlet and in each
outlet zone are less than 1.14% and 1.66%, respectively. The measurement error of gas velocity is less than 1.39%. The diameter of the inlet tube is 40 mm. The cross-section of the outlet of heat exchanger is composed of more than 1100 cold-flow micro-passages (the fin dimension of 9.5 × 2 × 0.3 mm3) and 1000 hot-flow micro-passages (the fin dimension of 6.5 × 2 × 0.3 mm3). It is hardly possible to measure flow velocity one by one for each micropassage. In order to quantitatively compare the two-phase flow distribution under different operation conditions, the outlet crosssection of the heat exchanger was divided into 30 zones (Fig. 2). It is defined into ordinate direction (parallel to the inlet tube) and crosswise direction (perpendicular to the inlet tube), which simplifies the measurement of the micro-passage flow. The dimension of each zone is 41.7 × 40 mm2, which is composed of several micro-passages and the flow distribution in it is assumed to be uniform.
2.2. The configuration of improved header Because the global size of the header is large compared with the inlet tube diameter, fluid tends to go preferentially into the channels in the center. The velocity distribution is basically symmetrical and decreases from the axis line to both ends. They are higher in the center and lower in the boundary. For the uniformity of flow distribution, different circulation area should be selected to the fluid field according to the velocity distribution. Based on the flow distribution of conventional header configuration, a baffle with small holes is put forward to be installed at the 1/2 height of the header symmetrically. The small holes are spotted in the baffle according to the velocity distribution of conventional header in CFD simulation and the perforated ratio is gradually increasing symmetrically from the central axis to the boundary [15,16].
Fig. 1. Schematic figure of experimental system.
118
S. Wang et al. / International Communications in Heat and Mass Transfer 37 (2010) 116–120
2.3. Evaluation of flow maldistribution A parameter S is introduced to evaluate the maldistribution, which is defined as follows, sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 N wave −wi 2 ∑ j = gas; liq; dry Sj = N−1 i = 1 wave θj =
wmax wmin
j = gas; liq; dry
1
2
The parameter Sj disclosures the dispersion degree of experimental results. Sgas and Sliq reflect distribution conditions of gas-phase and liquid-phase under different operation conditions. The smaller the value is, the more uniformly the fluid is distributed. Dryness shows local accumulation zone of single-phase, which is defined as the ratio of inlet gas-phase mass flow rate to that of inlet two-phase fluid. Sdry demonstrates the mixture uniformity of two-phase fluid. The smaller Sdry is, the more uniformly the two-phases are mixed. Where N stands for the zone number, wi stands for the measured value of zone i and wave stands for the average value of all the zones. 3. Experimental results and discussion 3.1. Distribution patterns of conventional header The distribution of two-phase flow of the conventional header is shown in Figs. 3 and 4 (Regas = 2400, Reliq = 3300, dryness = 7.20%). The distribution of two-phase flow is more nonuniform than singlephase and there are marked differences between them. The locations of maximum flow rate are different. The locations of gas-phase with lower flow rate are that of liquid-phase with larger flow rate, and vice versa. The flow rate of gas-phase at the outlet cross-section is between 0.002 and 0.219 m3/min. As shown in Fig. 3, the zones with larger flow rate are concentrated in the center part. The maximum flow appears in zone 18. The zones with very low flow rate are distributed near the boundary (zone 1 2 3 4 28 and 29). There are 12 zones with flow rate less than 0.700 m3/min, which occupy 40% of the whole zones. Seven zones have the flow rate above 0.140 m3/min, which takes about 23%. The ratio of the maximum flow rate to the minimum θgas is 129.2 and the maldistribution parameter Sgas is 0.585. The distribution of liquidphase is different from that of gas-phase (Fig. 4). The flow rate of liquid-phase is among 0.337 and 3.143 L/min. The maximum flow rate is located in zone 3. There are a few peak values in the crosswise directions. The number of zones with the flow rate less than 1.000 L/
Fig. 4. The outlet fluid distribution of liquid-phase.
min takes about 17% and the number with the flow rate above 2.0 L/ min takes about 20%. The ratio of the maximum flow rate to that of the minimum θliq is 9.33 and the maldistribution parameter Sliq is 0.383. With the interaction between the two-phases, the distribution of twophase is more nonuniform and complicate than that of single-phase. 3.2. Distribution patterns of improved header Figs. 3 and 4 also show the two-phase flow distribution of improved header (Regas = 2400, Reliq = 3300, dryness = 7.20%). The distribution uniformity is effectively enhanced compared with conventional header. For gas-phase, the flow rate near the center part decreases and the flow rate near the boundary increases accordingly (Fig. 3). The flow rate is among 0.032–0.142 m3/min. There are 8 zones with the flow rate less than 0.070 m3/min, which takes up 23.3% of the whole zones. There are only 2 zones with the flow rate above than 0.14 m3/min, which takes up 6.7% of the whole zones. Most of the flow rates are concentrated between 0.07 and 0.14 m3/min, which takes up 70% of the whole zones. The values of θgas and Sgas decrease to 4.44 and 0.380, respectively. For liquid-phase, the flow rate is among 0.730–2.460 L/min. The maximum flow rate decreased and the minimum value increased accordingly compared with that of conventional header (Fig. 4). There are 23 zones that have the flow rate among 1.0–2.5 m3/min, which takes up 76.7% of the whole zones. The values of θliq and Sliq decrease to 3.39 and 0.339, respectively. The uniformity of two-phase flow distribution in platefin heat exchanger is effectively enhanced by means of the configuration improvement. 3.3. Comparison of distribution of conventional header and improved header
Fig. 3. The outlet fluid distribution of gas-phase.
3.3.1. The comparison of two-phase flow distribution for different headers For conventional header, the distribution patterns of gas-phase are similar with the increase of Reliq when Regas is fixed to a certain value. In ordinate direction, the maximum flow rate is located near 60 mm and the minimum value is located near 180 mm. In crosswise direction, there are two peak values for gas-phase. When Reliq increases to 3300, more gas-phase flows to center part. The distribution nonuniformity of gas-phase in crosswise direction is worse than that in ordinate direction because of the header configuration parameters. In ordinate direction, more liquid-phase fluid flows to the right side as the increase of Reliq for higher inertia. In crosswise direction, the maximum flow rate is distributed on the left side while the minimum value is on the right
S. Wang et al. / International Communications in Heat and Mass Transfer 37 (2010) 116–120
side for the influence of distributors. The distribution uniformity of liquid-phase in crosswise direction is also worse compared with that in ordinate direction. The distribution uniformity of gas-phase deteriorates with the increase of Reliq but it improves with the increase of Regas. The uniformity of liquid-phase deteriorates with the decrease of Reliq and Regas (Table 1). For improved header, the distribution patterns of both gas-phase and liquid-phase in the two directions don't change much with the increase of Reliq when Regas is fixed to a certain value. It demonstrated that the effect of Reliq on the distribution of two-phase flow of improved header is limited, which is different from that of conventional header. For gasphase, there are two peak values in both of the crosswise and ordinate direction. The uniformity of gas-phase in crosswise direction is better than that in ordinate direction, which is opposite compared with that of conventional header. It demonstrates that the gas-phase uniformity in crosswise direction is obviously enhanced by the improved header. For liquid-phase, the larger flow rate is located near the center part especially in crosswise direction, which is opposite to that of gas-phase. When Reliq is 3300, the liquid-phase near the center part takes up about 70% of the whole liquid fluid. The distribution patterns of two-phase flow indicate that there is interaction between liquid-phase and gas-phase. And the perforated baffle also can influence the two-phase flow distribution obviously. The diameter of the holes near the center part on the baffle is the smallest. The liquid-phase flows through the small holes near the center part, which prevents gas-phase from flowing through them for higher viscosity of liquid-phase fluid and forces the gas-phase to flow through the ambient bigger holes. Among the operation conditions, the values of Sliq and Sgas decrease by 5.4–44.0% and 4.7–35.0% respectively compared with that of conventional one (Table 1). The configuration of improved header with perforated baffle can effectively improve the uniformity and stability of two-phase flow distribution in the heat exchanger.
3.3.2. The comparison of dryness distribution for different headers Table 1 also shows the dryness maldistribution parameter Sdry under different operating conditions. The values of Sdry all increase with Reliq and decrease with Regas. Resistance of liquid-phase may initiate phase separation of gas fluid under higher Reliq. Thus the uniformity of mixture between liquid-phase and gas-phase deteriorates, which results in the increase of Sdry. Muller et al. [11] found in their visual experiments that both phases flow uniformly upwards at
119
high gas flow rate and there is a continuous transition of flow patterns from annular to churn and slug flow with the decrease of Regas in a plate-fin heat exchanger passage. A further decrease in gas flow leads to the appearance of a large zone of mainly pure liquid that blocks parts of the passage for two-phase flow and the two-phases are distributed nonuniformly, accordingly. It is in accordance with the results of our experimental results. All of the values of improved header are smaller than that of the conventional. The results indicate that the improved header can greatly improve the uniformity of dryness distribution in two-phase flow. 4. Conclusions The two-phase distribution in plate-fin heat exchanger has been experimentally studied under different operation conditions. The experimental results indicate that the maldistribution of two-phase flow is very serious in conventional heat exchanger used in industry due to a non-proper header configuration. The distribution of two-phase flow is markedly different from that of single-phase flow. The distribution of two-phase flow is complicated because there is interaction between gas-phase and liquid-phase. The maldistribution parameter of gas-phase Sgas decreases with Regas while it increases with Reliq. The values of Sliq decrease with Reliq and Regas. The improved header with perforated baffle can effectively improve the uniformity of two-phase flow distribution in both ordinate and crosswise directions compared with conventional header. Under the operation conditions, the values of flow maldistribution parameter Sliq and Sgas decrease by 5.4–44.0% and 4.7–35.0%, respectively. The uniformity of dryness distribution in plate-fin heat exchanger is also enhanced by the improved configuration. Sdry decreases by 11.7–30.0%. The baffle is lower in cost and convenient in assembly, while the effect on the distribution uniformity of two-phase flow and dryness in heat exchanger core brought by the improved configuration is obvious. The conclusion of this paper is of great significance in the optimum design of plate-fin heat exchanger. Acknowledgements The paper is supported by the National Natural Science Foundation of China (NSFC 50676074), PCSIRT(IRT0746) and the Principal Foundation of XJTU (XJJ2008046), for which the authors are thankful.
Table 1 The comparison of maldistribution parameters. Regas
600
1200
1800
2400
Reliq
500 1000 1800 2400 3300 500 1000 1800 2400 3300 500 1000 1800 2400 3300 500 1000 1800 2400 3300
Dryness
11.60% 6.10% 3.40% 2.50% 1.90% 20.70% 11.60% 6.50% 5.00% 3.70% 28.20% 16.40% 9.50% 7.30% 5.50% 34.30% 20.70% 12.20% 9.50% 7.20%
Sgas
Sliq
Sdry
Conventional
Improved
Conventional
Improved
Conventional
Improved
0.953 0.967 1.018 1.184 1.415 0.713 0.812 0.893 0.945 0.976 0.508 0.609 0.614 0.705 0.733 0.443 0.462 0.474 0.478 0.585
0.779 0.803 0.951 1.083 1.186 0.679 0.766 0.790 0.794 0.849 0.415 0.439 0.461 0.469 0.516 0.367 0.391 0.369 0.387 0.380
1.183 1.006 0.752 0.870 0.768 0.998 0.904 0.894 0.720 0.687 0.811 0.658 0.547 0.530 0.502 0.722 0.723 0.445 0.399 0.383
1.021 0.951 0.688 0.651 0.618 0.869 0.767 0.557 0.536 0.454 0.636 0.517 0.450 0.354 0.349 0.447 0.405 0.375 0.338 0.339
0.657 0.690 0.827 0.852 0.870 0.455 0.597 0.628 0.665 0.666 0.405 0.561 0.589 0.595 0.655 0.399 0.444 0.508 0.511 0.644
0.483 0.508 0.579 0.597 0.617 0.398 0.445 0.506 0.506 0.574 0.294 0.400 0.416 0.431 0.506 0.295 0.349 0.404 0.451 0.473
120
S. Wang et al. / International Communications in Heat and Mass Transfer 37 (2010) 116–120
References [1] Ch. Ranganayakulu, K.N. Seetharamu, The combined effects of wall longitudinal heat conduction, inlet fluid flow nonuniformity and temperature nonuniformity in compact tube-fin heat exchangers: a finite element method, International Journal of Heat and Mass Transfer 42 (1999) 263–273. [2] R.K. Shah, A.L. London, Effects of nonuniformity passage in compact heat exchanger performance, Journal of Engineering for Power 102 (1980) 653–659. [3] B. Prabhakara Rao, P. Krishna Kumar, K. Das Sarit, Effects of flow distribution to the channels in the thermal performance of a plate heat exchanger, Chemical Engineering and Processing 41 (1) (2002) 41–58. [4] C. T'Joen, M. De Paepe, F. Vanhee, Heat exchanger behavior in non-uniform flow, Experimental Heat Transfer 19 (2006) 281–296. [5] W.M. Rohsenow, J.P. Hartnett, E.N. Ganic, Handbook of Heat Transfer, Applications. McGraw-Hill, New York, 1985. [6] J.P. Chiou, Thermal performance deterioration in crossflow heat exchanger due to flow nonunifromity, Journal of Heat Transfer 100 (1978) 580–587. [7] J.P.Chiou, The combined effects of maldistributions of inlet air temperature and induced flow nonuniformity on the performance of radiator, heat and oil cooler Int. Cong. SAE paper 850037,1985. [8] S. Lalot, P. Florent, S.K. Lang, A.E. Berglles, Flow maldsitribution in heat exchangers, Applied Thermal Engineering 19 (1999) 847–863.
[9] Ch. Ranganayakulu, K.N. Seetharamu, K.V. Sreevatsan, The effects of inlet fluid flow nonuniformity on thermal performance and pressure drops in crossflow plate-fin compact heat exchangers, International Journal of Heat and Mass Transfer 40 (1) (1997) 27–38. [10] J.B. Kitto, J.M. Robertson, Effect of maldistribution of flow on heat transfer equipment performance, Heat Transfer Engineering 10 (1) (1989) 18–25. [11] T. Muller-Menzel, T. Hecht, Plate-fin heat exchanger performance reduction in special two-phase flow conditions, Cryogenics 35 (5) (1995) 297–301. [12] T. Nobuyuki, A. Hitoshi, F. Terushige, U. Toshihiko, et al., Application of neutron radiography to visualization of cryogenic fluid boiling two-phase flows, Nuclear Instruments and Methods in Physics Research A 377 (1996) 174–176. [13] A.C. Muller, J.P. Chiou, Review of various types of flow maldistrubtion in heat exchangers, Heat Transfer Engineering 9 (2) (1988) 36–50. [14] A.M. Zhou, Effect of header Configuration on flow resistance and heat transfer performance in Plate-Fin Heat Exchanger, Xi'an Jiaotong University, 2006. [15] J. Wen, Y. Li, Study of flow distribution and its improvement on the header of plate-fin heat exchanger, Cryogenics 44 (2004) 823–831. [16] J. Wen, Y.Z. Li, S.M. Wang, A.M. Zhou, Experimental investigation of header configuration improvement in plate-fin heat exchanger, Applied Thermal Engineering 27 (11–12) (2007) 1761–1770.
Contents lists available at ScienceDirect
International Communications in Heat and Mass Transfer j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / i c h m t
Experimental investigation of header configuration on two-phase flow distribution in plate-fin heat exchanger☆ Simin Wang, Yanzhong Li, Jian Wen ⁎, Yansong Ma Department of Refrigeration and Cryogenics Engineering, School of Energy and Power Engineering, Xi'an Jiaotong University, Xi'an, 710049, China
a r t i c l e
i n f o
Available online 3 September 2009 Keyword: Plate-fin heat exchanger Two-phase flow Distribution Perforated baffle
a b s t r a c t The two-phase flow distribution in a plate-fin heat exchanger has been experimentally studied under different operation conditions. The results indicate that two-phase flow distribution is more complex and nonuniform than that of single-phase flow. The distribution uniformity of liquid-phase deteriorates with the decrease of Regas and Reliq. The distribution uniformity of gas-phase deteriorates with Reliq, but improves with Regas. The improved header with perforated baffle can effectively improve the uniformity of two-phase flow distribution and dryness distribution. The values of Sliq, Sgas and Sdry decrease by 5.4–44.0%, 4.7–35.0% and 11.7–30.0%, respectively. The conclusion is of great significance in the optimum design of plate-fin heat exchanger. © 2009 Elsevier Ltd. All rights reserved.
1. Introduction The use of plate-fin heat exchangers in industries around the world has increased considerably in recent years. They are widely used in process industries such as gas processing and petrochemical industries to exchange heat among more than two fluids with different supply temperatures due to their higher efficiency, more compact structure and lower costs than conventional two stream heat exchanger networks [1]. The common idealization in the basic plate-fin heat exchanger design theory is that the fluid is distributed uniformly at the inlet of the exchanger on each fluid channel throughout the core. However, in practice, flow maldistribution is more common and significantly reduces the idealized heat exchanger performance [2–4]. Heat exchangers in general and plate-fin heat exchangers in particular undergo decrease in performance due to flow maldistribution. The fluid tends to go preferentially into the channels that face the inlet tube because the inlet tube of plate-fin heat exchanger is small compared with the global size of the inlet header [5]. Much research has been done on the effects of flow nonuniformity on heat exchangers performance decrease. Based on the experimental data obtained from wind tunnel experiments, Chiou [6,7] set up a continuous flow distribution model and studied the thermal performance decrease in crossflow exchangers. Lalot et al. [8] presented the effect of flow nonuniformity on the performance of heat exchangers, based on the study of flow maldistribution in an experimental electrical heater. The results indicated that the flow maldistribution leads to a loss of effectiveness of about 25% for crossflow exchangers.
☆ Communicated by P. Cheng and W.Q. Tao. ⁎ Corresponding author. E-mail address: [email protected] (J. Wen). 0735-1933/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.icheatmasstransfer.2009.07.014
Ranganayakulu and Seetharamu [9] investigated the combined effects of wall longitudinal heat conduction, inlet flow uniformity and temperature nonuniformity on the thermal performance of a twofluid crossflow plate-fin heat exchanger using finite element method. The results showed that the performance may be reduced by 30% under extra nonuniform operating condition. For two-phase flow heat exchangers, the effect of flow maldistribution on performance is more serious, especially for the compact heat exchanger used in cryogenic plants. Since plate-fin heat exchangers are commonly used for small temperature difference, the slight maldistribution will result in serious decrease on heat transfer performance. It may even influence the mechanical behaviour of the heat exchangers in some cases [10]. Muller-Menzel et al. [11] observed various flow patterns in a plate-fin heat exchanger using a flow visualization rig. They found that at high gas mass fluxes both phases flow uniformly upwards. When decreasing the gas flow, a reversing slug flow becomes more and more pronounced. Nobuyuki Takenaka et al. [12] used real-time thermal neutron radiography to visualize boiling phenomena in a pool and boiling two-phase flow in an aluminum plate-fin heat exchanger. They all only investigated the phase distribution qualitatively inside only a single conduit because of the experimental limitation. The results of Muller and Chou [13] showed that the variation of dryness may result in the variation of pressure drop for two-phase flow, which will cause the maldistribution of two-phase flow in heat exchanger core. They also found that the unstable density-wave will cause surge of liquid-phase in heat exchanger. According to the previous studies, the effect of flow maldistribution on the thermal performance of a two-fluid crossflow heat exchanger is detrimental and not neglected. The use of a perforated baffle in the header can effectively enhance the distribution uniformity and heat transfer performance for single-phase flow. When air was adopted as
S. Wang et al. / International Communications in Heat and Mass Transfer 37 (2010) 116–120
117
Nomenclature N Re S w θ
the quantity of small zones Reynolds number maldistribution parameter measured value of flow rate or dryness ratio of the maximum value to the minimum
Subscripts ave average value dry dryness gas gas-phase i serial number of passage liq liquid-phase max maximum value min minimum value
Fig. 2. The division of outlet cross-section of heat exchanger.
the working fluid, the pressure drop of the improved heat exchanger increased by 277–1048 Pa, the heat transfer coefficient increases by 9.7–10.5% compared with the conventional one. The j and f factors increase by 8.0–11.5% and 15.6–45.6%, respectively [14]. No systematic study has been carried out for two-phase flow distribution. The objective of this paper is to experimentally study the two-phase flow distribution in plate-fin heat exchanger. The outlet distribution of both gas-phase and liquid-phase under different operation conditions was measured and the effects of inlet Reynolds number and header configuration on two-phase flow distribution were also investigated.
2 . Experimental system and evaluation of maldistribution 2.1. System description The schematic figure of experimental system is shown in Fig. 1. It is a multifunctional experimental system in which the experiments of both single-phase and two-phase flow distribution can be performed successfully. The experimental system consists of air loop, water loop and data acquisition system. Air is supplied by a compressor and is adjusted to required flow rate by different valves. The flow rate is measured by gas-turbine flowmeter. Water is pumped from tank to experimental tubes and its flow rate is measured by liquid-turbine flowmeter. Air and water are mixed evenly in the blender and then flow to the test part. The two-phase fluid at the outlet of heat exchanger flows into a switch box and separator to be separated into liquid and gas. The separated gas is measured by gas-turbine flowmeter and then is vented. The separated water flows downward into containers to be measured using weighing method. The measurement errors of the liquid velocity at the inlet and in each
outlet zone are less than 1.14% and 1.66%, respectively. The measurement error of gas velocity is less than 1.39%. The diameter of the inlet tube is 40 mm. The cross-section of the outlet of heat exchanger is composed of more than 1100 cold-flow micro-passages (the fin dimension of 9.5 × 2 × 0.3 mm3) and 1000 hot-flow micro-passages (the fin dimension of 6.5 × 2 × 0.3 mm3). It is hardly possible to measure flow velocity one by one for each micropassage. In order to quantitatively compare the two-phase flow distribution under different operation conditions, the outlet crosssection of the heat exchanger was divided into 30 zones (Fig. 2). It is defined into ordinate direction (parallel to the inlet tube) and crosswise direction (perpendicular to the inlet tube), which simplifies the measurement of the micro-passage flow. The dimension of each zone is 41.7 × 40 mm2, which is composed of several micro-passages and the flow distribution in it is assumed to be uniform.
2.2. The configuration of improved header Because the global size of the header is large compared with the inlet tube diameter, fluid tends to go preferentially into the channels in the center. The velocity distribution is basically symmetrical and decreases from the axis line to both ends. They are higher in the center and lower in the boundary. For the uniformity of flow distribution, different circulation area should be selected to the fluid field according to the velocity distribution. Based on the flow distribution of conventional header configuration, a baffle with small holes is put forward to be installed at the 1/2 height of the header symmetrically. The small holes are spotted in the baffle according to the velocity distribution of conventional header in CFD simulation and the perforated ratio is gradually increasing symmetrically from the central axis to the boundary [15,16].
Fig. 1. Schematic figure of experimental system.
118
S. Wang et al. / International Communications in Heat and Mass Transfer 37 (2010) 116–120
2.3. Evaluation of flow maldistribution A parameter S is introduced to evaluate the maldistribution, which is defined as follows, sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 N wave −wi 2 ∑ j = gas; liq; dry Sj = N−1 i = 1 wave θj =
wmax wmin
j = gas; liq; dry
1
2
The parameter Sj disclosures the dispersion degree of experimental results. Sgas and Sliq reflect distribution conditions of gas-phase and liquid-phase under different operation conditions. The smaller the value is, the more uniformly the fluid is distributed. Dryness shows local accumulation zone of single-phase, which is defined as the ratio of inlet gas-phase mass flow rate to that of inlet two-phase fluid. Sdry demonstrates the mixture uniformity of two-phase fluid. The smaller Sdry is, the more uniformly the two-phases are mixed. Where N stands for the zone number, wi stands for the measured value of zone i and wave stands for the average value of all the zones. 3. Experimental results and discussion 3.1. Distribution patterns of conventional header The distribution of two-phase flow of the conventional header is shown in Figs. 3 and 4 (Regas = 2400, Reliq = 3300, dryness = 7.20%). The distribution of two-phase flow is more nonuniform than singlephase and there are marked differences between them. The locations of maximum flow rate are different. The locations of gas-phase with lower flow rate are that of liquid-phase with larger flow rate, and vice versa. The flow rate of gas-phase at the outlet cross-section is between 0.002 and 0.219 m3/min. As shown in Fig. 3, the zones with larger flow rate are concentrated in the center part. The maximum flow appears in zone 18. The zones with very low flow rate are distributed near the boundary (zone 1 2 3 4 28 and 29). There are 12 zones with flow rate less than 0.700 m3/min, which occupy 40% of the whole zones. Seven zones have the flow rate above 0.140 m3/min, which takes about 23%. The ratio of the maximum flow rate to the minimum θgas is 129.2 and the maldistribution parameter Sgas is 0.585. The distribution of liquidphase is different from that of gas-phase (Fig. 4). The flow rate of liquid-phase is among 0.337 and 3.143 L/min. The maximum flow rate is located in zone 3. There are a few peak values in the crosswise directions. The number of zones with the flow rate less than 1.000 L/
Fig. 4. The outlet fluid distribution of liquid-phase.
min takes about 17% and the number with the flow rate above 2.0 L/ min takes about 20%. The ratio of the maximum flow rate to that of the minimum θliq is 9.33 and the maldistribution parameter Sliq is 0.383. With the interaction between the two-phases, the distribution of twophase is more nonuniform and complicate than that of single-phase. 3.2. Distribution patterns of improved header Figs. 3 and 4 also show the two-phase flow distribution of improved header (Regas = 2400, Reliq = 3300, dryness = 7.20%). The distribution uniformity is effectively enhanced compared with conventional header. For gas-phase, the flow rate near the center part decreases and the flow rate near the boundary increases accordingly (Fig. 3). The flow rate is among 0.032–0.142 m3/min. There are 8 zones with the flow rate less than 0.070 m3/min, which takes up 23.3% of the whole zones. There are only 2 zones with the flow rate above than 0.14 m3/min, which takes up 6.7% of the whole zones. Most of the flow rates are concentrated between 0.07 and 0.14 m3/min, which takes up 70% of the whole zones. The values of θgas and Sgas decrease to 4.44 and 0.380, respectively. For liquid-phase, the flow rate is among 0.730–2.460 L/min. The maximum flow rate decreased and the minimum value increased accordingly compared with that of conventional header (Fig. 4). There are 23 zones that have the flow rate among 1.0–2.5 m3/min, which takes up 76.7% of the whole zones. The values of θliq and Sliq decrease to 3.39 and 0.339, respectively. The uniformity of two-phase flow distribution in platefin heat exchanger is effectively enhanced by means of the configuration improvement. 3.3. Comparison of distribution of conventional header and improved header
Fig. 3. The outlet fluid distribution of gas-phase.
3.3.1. The comparison of two-phase flow distribution for different headers For conventional header, the distribution patterns of gas-phase are similar with the increase of Reliq when Regas is fixed to a certain value. In ordinate direction, the maximum flow rate is located near 60 mm and the minimum value is located near 180 mm. In crosswise direction, there are two peak values for gas-phase. When Reliq increases to 3300, more gas-phase flows to center part. The distribution nonuniformity of gas-phase in crosswise direction is worse than that in ordinate direction because of the header configuration parameters. In ordinate direction, more liquid-phase fluid flows to the right side as the increase of Reliq for higher inertia. In crosswise direction, the maximum flow rate is distributed on the left side while the minimum value is on the right
S. Wang et al. / International Communications in Heat and Mass Transfer 37 (2010) 116–120
side for the influence of distributors. The distribution uniformity of liquid-phase in crosswise direction is also worse compared with that in ordinate direction. The distribution uniformity of gas-phase deteriorates with the increase of Reliq but it improves with the increase of Regas. The uniformity of liquid-phase deteriorates with the decrease of Reliq and Regas (Table 1). For improved header, the distribution patterns of both gas-phase and liquid-phase in the two directions don't change much with the increase of Reliq when Regas is fixed to a certain value. It demonstrated that the effect of Reliq on the distribution of two-phase flow of improved header is limited, which is different from that of conventional header. For gasphase, there are two peak values in both of the crosswise and ordinate direction. The uniformity of gas-phase in crosswise direction is better than that in ordinate direction, which is opposite compared with that of conventional header. It demonstrates that the gas-phase uniformity in crosswise direction is obviously enhanced by the improved header. For liquid-phase, the larger flow rate is located near the center part especially in crosswise direction, which is opposite to that of gas-phase. When Reliq is 3300, the liquid-phase near the center part takes up about 70% of the whole liquid fluid. The distribution patterns of two-phase flow indicate that there is interaction between liquid-phase and gas-phase. And the perforated baffle also can influence the two-phase flow distribution obviously. The diameter of the holes near the center part on the baffle is the smallest. The liquid-phase flows through the small holes near the center part, which prevents gas-phase from flowing through them for higher viscosity of liquid-phase fluid and forces the gas-phase to flow through the ambient bigger holes. Among the operation conditions, the values of Sliq and Sgas decrease by 5.4–44.0% and 4.7–35.0% respectively compared with that of conventional one (Table 1). The configuration of improved header with perforated baffle can effectively improve the uniformity and stability of two-phase flow distribution in the heat exchanger.
3.3.2. The comparison of dryness distribution for different headers Table 1 also shows the dryness maldistribution parameter Sdry under different operating conditions. The values of Sdry all increase with Reliq and decrease with Regas. Resistance of liquid-phase may initiate phase separation of gas fluid under higher Reliq. Thus the uniformity of mixture between liquid-phase and gas-phase deteriorates, which results in the increase of Sdry. Muller et al. [11] found in their visual experiments that both phases flow uniformly upwards at
119
high gas flow rate and there is a continuous transition of flow patterns from annular to churn and slug flow with the decrease of Regas in a plate-fin heat exchanger passage. A further decrease in gas flow leads to the appearance of a large zone of mainly pure liquid that blocks parts of the passage for two-phase flow and the two-phases are distributed nonuniformly, accordingly. It is in accordance with the results of our experimental results. All of the values of improved header are smaller than that of the conventional. The results indicate that the improved header can greatly improve the uniformity of dryness distribution in two-phase flow. 4. Conclusions The two-phase distribution in plate-fin heat exchanger has been experimentally studied under different operation conditions. The experimental results indicate that the maldistribution of two-phase flow is very serious in conventional heat exchanger used in industry due to a non-proper header configuration. The distribution of two-phase flow is markedly different from that of single-phase flow. The distribution of two-phase flow is complicated because there is interaction between gas-phase and liquid-phase. The maldistribution parameter of gas-phase Sgas decreases with Regas while it increases with Reliq. The values of Sliq decrease with Reliq and Regas. The improved header with perforated baffle can effectively improve the uniformity of two-phase flow distribution in both ordinate and crosswise directions compared with conventional header. Under the operation conditions, the values of flow maldistribution parameter Sliq and Sgas decrease by 5.4–44.0% and 4.7–35.0%, respectively. The uniformity of dryness distribution in plate-fin heat exchanger is also enhanced by the improved configuration. Sdry decreases by 11.7–30.0%. The baffle is lower in cost and convenient in assembly, while the effect on the distribution uniformity of two-phase flow and dryness in heat exchanger core brought by the improved configuration is obvious. The conclusion of this paper is of great significance in the optimum design of plate-fin heat exchanger. Acknowledgements The paper is supported by the National Natural Science Foundation of China (NSFC 50676074), PCSIRT(IRT0746) and the Principal Foundation of XJTU (XJJ2008046), for which the authors are thankful.
Table 1 The comparison of maldistribution parameters. Regas
600
1200
1800
2400
Reliq
500 1000 1800 2400 3300 500 1000 1800 2400 3300 500 1000 1800 2400 3300 500 1000 1800 2400 3300
Dryness
11.60% 6.10% 3.40% 2.50% 1.90% 20.70% 11.60% 6.50% 5.00% 3.70% 28.20% 16.40% 9.50% 7.30% 5.50% 34.30% 20.70% 12.20% 9.50% 7.20%
Sgas
Sliq
Sdry
Conventional
Improved
Conventional
Improved
Conventional
Improved
0.953 0.967 1.018 1.184 1.415 0.713 0.812 0.893 0.945 0.976 0.508 0.609 0.614 0.705 0.733 0.443 0.462 0.474 0.478 0.585
0.779 0.803 0.951 1.083 1.186 0.679 0.766 0.790 0.794 0.849 0.415 0.439 0.461 0.469 0.516 0.367 0.391 0.369 0.387 0.380
1.183 1.006 0.752 0.870 0.768 0.998 0.904 0.894 0.720 0.687 0.811 0.658 0.547 0.530 0.502 0.722 0.723 0.445 0.399 0.383
1.021 0.951 0.688 0.651 0.618 0.869 0.767 0.557 0.536 0.454 0.636 0.517 0.450 0.354 0.349 0.447 0.405 0.375 0.338 0.339
0.657 0.690 0.827 0.852 0.870 0.455 0.597 0.628 0.665 0.666 0.405 0.561 0.589 0.595 0.655 0.399 0.444 0.508 0.511 0.644
0.483 0.508 0.579 0.597 0.617 0.398 0.445 0.506 0.506 0.574 0.294 0.400 0.416 0.431 0.506 0.295 0.349 0.404 0.451 0.473
120
S. Wang et al. / International Communications in Heat and Mass Transfer 37 (2010) 116–120
References [1] Ch. Ranganayakulu, K.N. Seetharamu, The combined effects of wall longitudinal heat conduction, inlet fluid flow nonuniformity and temperature nonuniformity in compact tube-fin heat exchangers: a finite element method, International Journal of Heat and Mass Transfer 42 (1999) 263–273. [2] R.K. Shah, A.L. London, Effects of nonuniformity passage in compact heat exchanger performance, Journal of Engineering for Power 102 (1980) 653–659. [3] B. Prabhakara Rao, P. Krishna Kumar, K. Das Sarit, Effects of flow distribution to the channels in the thermal performance of a plate heat exchanger, Chemical Engineering and Processing 41 (1) (2002) 41–58. [4] C. T'Joen, M. De Paepe, F. Vanhee, Heat exchanger behavior in non-uniform flow, Experimental Heat Transfer 19 (2006) 281–296. [5] W.M. Rohsenow, J.P. Hartnett, E.N. Ganic, Handbook of Heat Transfer, Applications. McGraw-Hill, New York, 1985. [6] J.P. Chiou, Thermal performance deterioration in crossflow heat exchanger due to flow nonunifromity, Journal of Heat Transfer 100 (1978) 580–587. [7] J.P.Chiou, The combined effects of maldistributions of inlet air temperature and induced flow nonuniformity on the performance of radiator, heat and oil cooler Int. Cong. SAE paper 850037,1985. [8] S. Lalot, P. Florent, S.K. Lang, A.E. Berglles, Flow maldsitribution in heat exchangers, Applied Thermal Engineering 19 (1999) 847–863.
[9] Ch. Ranganayakulu, K.N. Seetharamu, K.V. Sreevatsan, The effects of inlet fluid flow nonuniformity on thermal performance and pressure drops in crossflow plate-fin compact heat exchangers, International Journal of Heat and Mass Transfer 40 (1) (1997) 27–38. [10] J.B. Kitto, J.M. Robertson, Effect of maldistribution of flow on heat transfer equipment performance, Heat Transfer Engineering 10 (1) (1989) 18–25. [11] T. Muller-Menzel, T. Hecht, Plate-fin heat exchanger performance reduction in special two-phase flow conditions, Cryogenics 35 (5) (1995) 297–301. [12] T. Nobuyuki, A. Hitoshi, F. Terushige, U. Toshihiko, et al., Application of neutron radiography to visualization of cryogenic fluid boiling two-phase flows, Nuclear Instruments and Methods in Physics Research A 377 (1996) 174–176. [13] A.C. Muller, J.P. Chiou, Review of various types of flow maldistrubtion in heat exchangers, Heat Transfer Engineering 9 (2) (1988) 36–50. [14] A.M. Zhou, Effect of header Configuration on flow resistance and heat transfer performance in Plate-Fin Heat Exchanger, Xi'an Jiaotong University, 2006. [15] J. Wen, Y. Li, Study of flow distribution and its improvement on the header of plate-fin heat exchanger, Cryogenics 44 (2004) 823–831. [16] J. Wen, Y.Z. Li, S.M. Wang, A.M. Zhou, Experimental investigation of header configuration improvement in plate-fin heat exchanger, Applied Thermal Engineering 27 (11–12) (2007) 1761–1770.