论文部分内容阅读
Heat transfer, friction factor and enhancement efficiency characteristics in an elliptic tube fitted with elliptic rings have been investigated experimentally. In the experiments, air was used as the tested fluid with a Reynolds number range of 10000 to 32464. The experimental results show a considerable increase in friction factor and heat transfer over the plain tube under the same operation conditions. Over the range investigated, the Nusselt numbers for both employed enhancement devices with different pitches are found to be higher than that of the plain tube. It was found that the best overall enhancement was achieved with pitch = 3d. The results obtained are correlated in the form of Nusselt number and friction factor as a function of Reynolds number and pitches. The results were compared with circular tubes have the same test conditions to show the difference between the circular and elliptic tubes.
Key words: Heat transfer enhancement; Elliptic tube
Ibrahim, E. Z. (2012). Heat Transfer Enhancement Inside Elliptic Tube by Means of Rings Inserts. Energy Science and Technology, 4(2),-0. Available from: http://www.cscanada.net/index.php/est/article/view/10.3968/j.est.1923847920120402.549
DOI: http://dx.doi.org/10.3968/j.est.1923847920120402.549
INTRODUCTION
The development of high-performance thermal systems still has a considerable interest among the researchers to improve heat transfer. The conventional heat exchangers are improved by means of various augmentation techniques with emphasis on surface enhancements technique. Augmented surfaces can create one or more combinations of the following conditions that are favorable for the increase in heat transfer rate with an undesirable rise of friction: 1) disruption of the development of boundary layer and increase of the turbulence intensity, and 2) increase in heat transfer area. The heat transfer enhancement technology has been improved and widely used in heat exchanger applications; such as refrigeration, automotives, process industry, chemical industry, etc. One of the widely used heat transfer enhancement technique is inserting different shaped elements with different geometries in channel flow (Khaled, 2007; Shoji et al., 2003; Zimparov, 2001).
Yakut and Sahin (Yakut et al., 2004) studied the heat transfer and friction characteristics in a uniform heat flux fitted with conical-ring turbulators used to provide reverse/turbulent flows in each module of the conical rings. Therefore, significant improvement of heat transfer along the tube wall was achieved. Promvonge and Eiamsa-ard (Promvonge et al., 2006) investigated the enhancements of heat transfer in a uniform heat-flux circular tube fitted with conical-nozzles and swirl generator. In their research, the conical-nozzles were placed in a test tube with three different pitch ratios of conical-nozzles, apart from the snail mounted at the tube inlet. The use of the conical-nozzle in conjunction with the snail entrance led to maximum heat transfer rate up to 316% over the plain tube besides a substantial increase in pressure loss as well. Eiamsa-ard and Promvonge (Eiamsa-ard et al., 2006) investigated the influences of V-nozzle inserts on heat transfer, flow friction, and enhancement efficiency characteristics in a uniform heat-flux tube with three pitch ratios. They found that the use of V-nozzles led to a considerable increase in heat transfer rate and a maximum gain of 1.19 on enhancement efficiency was obtained for the smallest pitch ratio used at low Reynolds number. This indicates the crucial effect of the reverse/recirculation flow can promote the heat transfer rate in tubes.
Smithberg and Landis (Smithberg et al., 1964) studied friction and forced convection heat transfer characteristics in tubes fitted with twisted tape swirl generators, and presented a correlation for predicting Nusselt number and friction factor. Date and Singham (Date et al., 1972), and Date (Date, 1974) reported the prediction of fully developed flow in a tube containing a twisted tape. Hong and Bergles (Hong at al., 1976) correlated heat transfer and pressure drop date for twisted tape inserts for uniform wall temperature conditions using water as working fluids in laminar flow. Eiamsa-ard, and Promvonge (Eiamsa-ard at al., 2005) reported the enhancement of the heat transfer in a tube with regularly spaced and full-length helical tape swirl generators, and concluded that the full-length helical tape with rod provides higher heat transfer rate than without rod by about 10%.
Chang et al. (Chang et al., 2007) presented an original experimental study on compound heat transfer enhancement in a tube fitted with serrated twisted tape. A set of empirical correlations that permits the evaluation of the Nusselt number and the fanning friction factor in the developed flow region for the tubes fitted with smooth and serrated twisted tapes was generated for engineering applications. Shaikh (Shaikh et al., 2007) studied the effect of baffle size and orientation on the heat transfer enhancement. The results showed that for the vertical baffle, an increase in the baffle height causes a substantial increase in the Nusselt number but the pressure loss is very significant. For the inclined baffles, the Nusselt number enhancement is almost independent of the baffle inclination angle. Naphon (Naphon, 2006) presented the heat transfer characteristics and pressure drop results of the horizontal double tubes with coil-wire insert. It was seen that the heat transfer rate and heat transfer coefficient depend directly on the mass flow rates of hot and cold water. They added that the effect of coil-wire insert on the enhancement of heat transfer tends to decrease as Reynolds number increases. Several investigations were carried out to determine the effect of the coiled wire or the twisted tape elements on heat transfer and friction factor for a long time studied by Garc??a et al. (Garc??a et al., 2007) and Ozceyhan (Ozceyhan, 2005) respectively. This is because the wire coil or twisted tape inserted in the tube created swirling flows that modify the near wall velocity profile due to the various velocities distributions in the vortex core.
Kim et al. (Kim et al.,2001) investigated the flow pattern, void fraction and slug rise velocity on counter current two-phase flow in a vertical round tube with coiled wire inserts. They observed that the slug rise velocity and void fraction in a vertical round tube is higher for a coiled wire insert than that in a smooth tube. Ozceyhan et al. (Ozceyhan et al., 2001) conducted a numerical study for investigating the heat transfer enhancement in a tube with the circular cross sectional rings. The rings were inserted near the tube wall. Five different pitches were considered as p = d/2, p = d, p = 3d/2, p = 2d and p = 3d. A uniform heat flux was applied to the external surface of the tube and air was selected as working fluid. The results show that, Nusselt number increases and friction factor decreases with increasing Reynolds number and the highest Nusselt number and friction factor is obtained in the case of p = d/2. Heat transfer in flat tubes by means of helical screw-tape inserts was studied by Ibrahim (Ibrahim, 2011). The study showed that, the Nusslet number (Nu) and friction factor (f) decrease with the increase of spacer length or twist ratio for flat tube.
As mentioned above, the literature survey on investigations of different types of tube inserts indicates that these inserts generally attached on the tube walls in order to enhance the heat transfer and increase the effective heat transfer area. This work differs from those in the literature by attaching the elliptic ring inserts separated from the elliptic tube wall, thus the effective heat transfer area was not increased and heat transfer enhancement was achieved.
The objective of this paper is to study the heat transfer and flow characteristics in a horizontal elliptic tube with elliptic cross sectional ring inserts. The rings inserted near the tube wall and the heat transfer enhancement is investigated related with the effect of spacing between the rings. Five different spacings between the rings are considered as p = d/2, p = d, p = 3d/2, p = 2d and p = 3d. A uniform heat flux will be applied through the external surface of the elliptic tube and the air passes through it. 1. EXPERIMENTAL APPRATUS
The experiments were carried out in an open-loop experimental facility as shown in Figure 1. The loop consisted of a 3 kW blower, orifice meter to measure the flow rate, and the heat transfer test section. The outer surface of the elliptic tube is covered with an electric insulating tape on which nickel-chrome wire of 0.4 mm was uniformly wound to form the main heater as shown in Figure 2. The main heater is covered with an asbestos layer of 55 mm thickness, on which another nickel-chrome wire of 0.4 mm was uniformly wound to form a guard heater. The Guard heater is covered with a 40 mm thick asbestos layer. The elliptic tube ends are connected to transient sections to convert from circular to elliptic sections. These transient sections let good connections between the elliptic tube and the experiment loop. Two pairs of thermocouples are installed in the asbestos layer between the main heater and the guard heater. The thermocouples of each pair are fixed on the same radial line. The input to the guard heater is adjusted so that, at the steady state, the readings of the thermocouples of each pair became practically the same. Then all the heat generated by the main heater is flowing inward to the elliptic tube. The terminals of the wire are connected to a variac AC transformer, which is used to vary the voltage of the AC current passing through the heating coil. The test-section is elliptic tube and made of copper having a 43 mm major axis and 15 mm minor axis (hydraulic diameter = 27.72 mm) , 1250 mm long (L) and 1.5 mm thick (t) as shown in Figure 3. A copper elliptic rings have the same inner dimensions of the elliptic tube are fixed on the inside surface of the elliptic tube. The number of these rings depends on the required pitch for each case. The inner and outer temperatures of the bulk air were measured at certain points with a multi-channel temperature measurement unit in conjunction with the thermocouple type-K. Twenty thermocouples were tapped on the local wall of the tube and the thermocouples were placed around the tube to measure the circumferential temperature variation. The mean local wall temperature was determined by means of calculations based on the reading of type-K thermocouples.
Key words: Heat transfer enhancement; Elliptic tube
Ibrahim, E. Z. (2012). Heat Transfer Enhancement Inside Elliptic Tube by Means of Rings Inserts. Energy Science and Technology, 4(2),
DOI: http://dx.doi.org/10.3968/j.est.1923847920120402.549
INTRODUCTION
The development of high-performance thermal systems still has a considerable interest among the researchers to improve heat transfer. The conventional heat exchangers are improved by means of various augmentation techniques with emphasis on surface enhancements technique. Augmented surfaces can create one or more combinations of the following conditions that are favorable for the increase in heat transfer rate with an undesirable rise of friction: 1) disruption of the development of boundary layer and increase of the turbulence intensity, and 2) increase in heat transfer area. The heat transfer enhancement technology has been improved and widely used in heat exchanger applications; such as refrigeration, automotives, process industry, chemical industry, etc. One of the widely used heat transfer enhancement technique is inserting different shaped elements with different geometries in channel flow (Khaled, 2007; Shoji et al., 2003; Zimparov, 2001).
Yakut and Sahin (Yakut et al., 2004) studied the heat transfer and friction characteristics in a uniform heat flux fitted with conical-ring turbulators used to provide reverse/turbulent flows in each module of the conical rings. Therefore, significant improvement of heat transfer along the tube wall was achieved. Promvonge and Eiamsa-ard (Promvonge et al., 2006) investigated the enhancements of heat transfer in a uniform heat-flux circular tube fitted with conical-nozzles and swirl generator. In their research, the conical-nozzles were placed in a test tube with three different pitch ratios of conical-nozzles, apart from the snail mounted at the tube inlet. The use of the conical-nozzle in conjunction with the snail entrance led to maximum heat transfer rate up to 316% over the plain tube besides a substantial increase in pressure loss as well. Eiamsa-ard and Promvonge (Eiamsa-ard et al., 2006) investigated the influences of V-nozzle inserts on heat transfer, flow friction, and enhancement efficiency characteristics in a uniform heat-flux tube with three pitch ratios. They found that the use of V-nozzles led to a considerable increase in heat transfer rate and a maximum gain of 1.19 on enhancement efficiency was obtained for the smallest pitch ratio used at low Reynolds number. This indicates the crucial effect of the reverse/recirculation flow can promote the heat transfer rate in tubes.
Smithberg and Landis (Smithberg et al., 1964) studied friction and forced convection heat transfer characteristics in tubes fitted with twisted tape swirl generators, and presented a correlation for predicting Nusselt number and friction factor. Date and Singham (Date et al., 1972), and Date (Date, 1974) reported the prediction of fully developed flow in a tube containing a twisted tape. Hong and Bergles (Hong at al., 1976) correlated heat transfer and pressure drop date for twisted tape inserts for uniform wall temperature conditions using water as working fluids in laminar flow. Eiamsa-ard, and Promvonge (Eiamsa-ard at al., 2005) reported the enhancement of the heat transfer in a tube with regularly spaced and full-length helical tape swirl generators, and concluded that the full-length helical tape with rod provides higher heat transfer rate than without rod by about 10%.
Chang et al. (Chang et al., 2007) presented an original experimental study on compound heat transfer enhancement in a tube fitted with serrated twisted tape. A set of empirical correlations that permits the evaluation of the Nusselt number and the fanning friction factor in the developed flow region for the tubes fitted with smooth and serrated twisted tapes was generated for engineering applications. Shaikh (Shaikh et al., 2007) studied the effect of baffle size and orientation on the heat transfer enhancement. The results showed that for the vertical baffle, an increase in the baffle height causes a substantial increase in the Nusselt number but the pressure loss is very significant. For the inclined baffles, the Nusselt number enhancement is almost independent of the baffle inclination angle. Naphon (Naphon, 2006) presented the heat transfer characteristics and pressure drop results of the horizontal double tubes with coil-wire insert. It was seen that the heat transfer rate and heat transfer coefficient depend directly on the mass flow rates of hot and cold water. They added that the effect of coil-wire insert on the enhancement of heat transfer tends to decrease as Reynolds number increases. Several investigations were carried out to determine the effect of the coiled wire or the twisted tape elements on heat transfer and friction factor for a long time studied by Garc??a et al. (Garc??a et al., 2007) and Ozceyhan (Ozceyhan, 2005) respectively. This is because the wire coil or twisted tape inserted in the tube created swirling flows that modify the near wall velocity profile due to the various velocities distributions in the vortex core.
Kim et al. (Kim et al.,2001) investigated the flow pattern, void fraction and slug rise velocity on counter current two-phase flow in a vertical round tube with coiled wire inserts. They observed that the slug rise velocity and void fraction in a vertical round tube is higher for a coiled wire insert than that in a smooth tube. Ozceyhan et al. (Ozceyhan et al., 2001) conducted a numerical study for investigating the heat transfer enhancement in a tube with the circular cross sectional rings. The rings were inserted near the tube wall. Five different pitches were considered as p = d/2, p = d, p = 3d/2, p = 2d and p = 3d. A uniform heat flux was applied to the external surface of the tube and air was selected as working fluid. The results show that, Nusselt number increases and friction factor decreases with increasing Reynolds number and the highest Nusselt number and friction factor is obtained in the case of p = d/2. Heat transfer in flat tubes by means of helical screw-tape inserts was studied by Ibrahim (Ibrahim, 2011). The study showed that, the Nusslet number (Nu) and friction factor (f) decrease with the increase of spacer length or twist ratio for flat tube.
As mentioned above, the literature survey on investigations of different types of tube inserts indicates that these inserts generally attached on the tube walls in order to enhance the heat transfer and increase the effective heat transfer area. This work differs from those in the literature by attaching the elliptic ring inserts separated from the elliptic tube wall, thus the effective heat transfer area was not increased and heat transfer enhancement was achieved.
The objective of this paper is to study the heat transfer and flow characteristics in a horizontal elliptic tube with elliptic cross sectional ring inserts. The rings inserted near the tube wall and the heat transfer enhancement is investigated related with the effect of spacing between the rings. Five different spacings between the rings are considered as p = d/2, p = d, p = 3d/2, p = 2d and p = 3d. A uniform heat flux will be applied through the external surface of the elliptic tube and the air passes through it. 1. EXPERIMENTAL APPRATUS
The experiments were carried out in an open-loop experimental facility as shown in Figure 1. The loop consisted of a 3 kW blower, orifice meter to measure the flow rate, and the heat transfer test section. The outer surface of the elliptic tube is covered with an electric insulating tape on which nickel-chrome wire of 0.4 mm was uniformly wound to form the main heater as shown in Figure 2. The main heater is covered with an asbestos layer of 55 mm thickness, on which another nickel-chrome wire of 0.4 mm was uniformly wound to form a guard heater. The Guard heater is covered with a 40 mm thick asbestos layer. The elliptic tube ends are connected to transient sections to convert from circular to elliptic sections. These transient sections let good connections between the elliptic tube and the experiment loop. Two pairs of thermocouples are installed in the asbestos layer between the main heater and the guard heater. The thermocouples of each pair are fixed on the same radial line. The input to the guard heater is adjusted so that, at the steady state, the readings of the thermocouples of each pair became practically the same. Then all the heat generated by the main heater is flowing inward to the elliptic tube. The terminals of the wire are connected to a variac AC transformer, which is used to vary the voltage of the AC current passing through the heating coil. The test-section is elliptic tube and made of copper having a 43 mm major axis and 15 mm minor axis (hydraulic diameter = 27.72 mm) , 1250 mm long (L) and 1.5 mm thick (t) as shown in Figure 3. A copper elliptic rings have the same inner dimensions of the elliptic tube are fixed on the inside surface of the elliptic tube. The number of these rings depends on the required pitch for each case. The inner and outer temperatures of the bulk air were measured at certain points with a multi-channel temperature measurement unit in conjunction with the thermocouple type-K. Twenty thermocouples were tapped on the local wall of the tube and the thermocouples were placed around the tube to measure the circumferential temperature variation. The mean local wall temperature was determined by means of calculations based on the reading of type-K thermocouples.