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1.
Wire coil inserted tubes are important in engineering applications. The conjugate heat transfer and thermal stress, which is induced by temperature differences in the wire coil inserted tube sheet of heat transfer equipment, were studied numerically in the paper. Three different wire coil inserts, which have different pitches, are considered. The smooth tube is also considered for comparison. Uniform heat flux was applied from the external surface of the tube. Water has been used as fluid. The energy and governing flow equations were solved using a finite difference scheme. The finite element method (FEM) was used to compute the thermal stress fields. The effect of the different wire coils on the thermal stress has been discussed in terms of the results extracted from the FEM. It was found that the maximum thermal stresses ratio occurred in the case of p = 2d for 3 m/s mean water velocity. Some designs to reduce the thermal stress in the wire coiled tube sheet were suggested.  相似文献   

2.
Helical-wire-coils fitted inside a round tube have been experimentally studied in order to characterize their thermohydraulic behaviour in laminar, transition and turbulent flow. By using water and water–propylene glycol mixtures at different temperatures, a wide range of flow conditions have been covered: Reynolds numbers from 80 to 90,000 and Prandtl numbers from 2.8 to 150. Six wire coils were tested within a geometrical range of helical pitch 1.17 < p/d < 2.68 and wire diameter 0.07 < e/d < 0.10. Experimental correlations of Fanning friction factor and Nusselt number as functions of flow and dimensionless geometric parameters have been proposed. Results have shown that in turbulent flow wire coils increase pressure drop up to nine times and heat transfer up to four times compared to the empty smooth tube. At low Reynolds numbers, wire coils behave as a smooth tube but accelerate transition to critical Reynolds numbers down to 700. Within the transition region, if wire coils are fitted inside a smooth tube heat exchanger, heat transfer rate can be increased up to 200% keeping pumping power constant. Wire coil inserts offer their best performance within the transition region where they show a considerable advantage over other enhancement techniques.  相似文献   

3.
An empirical setup has been established to study heat transfer and pressure drop characteristics during condensation of R600a, a hydrocarbon refrigerant, in a horizontal plain tube and different flattened channels. Round copper tubes of 8.7 mm I.D. were deformed into flattened channels with different interior heights of 6.7 mm, 5.2 mm and 3.1 mm as test sections. The test conditions include heat flux of 17 kw/m2, mass velocity in the range of 154.8–265.4 kg/m2s and vapor quality variation from approximately 10% to 80%. Results indicate that flattening the tubes causes significant enhancement of heat transfer coefficient which is also accompanied by simultaneous augmentation in flow pressure drop. Therefore, the overall performance of the flattened tubes with respect to heat transfer enhancement considering the pressure drop penalty is analyzed. It is concluded that the flattened tube with 5.2 mm inner height tube has the best overall performance. Due to the failure of pre-existing correlations for round tube condensation heat transfer, a new correlation is proposed which predicts 90% of the entire data within ± 17% error.  相似文献   

4.
《Applied Thermal Engineering》2007,27(16):2713-2726
Enhanced heat transfer surfaces are used in heat exchangers to improve performance and to decrease system volume and cost. In-tube heat transfer enhancement usually takes the form of either micro-fin tubes (of the helical micro-fin or herringbone varieties), or of helical wire inserts. Despite a substantial increase in heat transfer, these devices also cause non-negligible pressure drops.By making use of well-proven flow pattern maps for smooth tubes and the new ones for smooth and enhanced tubes, it is shown from the refrigerant condensation data that flow patterns have a strong influence on heat transfer and pressure drop. This is done for data obtained from in-tube condensation experiments for mass fluxes ranging from 300 to 800 kg/m2 s at a saturation temperature of 40 °C, for refrigerants R-22, R-134a, and R-407C. The flow regimes, pressure drops, heat transfer coefficients, and the overall performance of three different tubes, namely a smooth-, 18° helical micro-fin-, and a herringbone micro-fin tube (each having a nominal diameter of 9.51 mm), are presented and compared to the performance of smooth tubes with helical wire inserts (with pitches of 5 mm, 7.77 mm and 11 mm corresponding to helical angles of 78.2°, 72°, and 65.3°, respectively).  相似文献   

5.
This study investigates heat transfer and flow characteristics of water flowing through horizontal internally grooved tubes. The test tubes consisted of one smooth tube, one straight grooved tube, and four grooved tubes with different pitches. All test tubes were made from type 304 stainless steel. The length and inner diameter of the test tube were 2 m and 7.1 mm, respectively. Water was used as working fluid, heated by DC power supply under constant heat flux condition. The test runs were performed at average fluid temperature of 25 °C, heat flux of 3.5 kW/m2, and Reynolds number range from 4000 to 10,000. The effect of grooved pitch on heat transfer and pressure drop was also investigated. The performance of the grooved tubes was discussed in terms of thermal enhancement factor. The results showed that the thermal enhancement factor obtained from groove tubes is about 1.4 to 2.2 for a pitch of 0.5 in.; 1.1 to 1.3 for pitches of 8, 10, and 12 in., respectively; and 0.8 to 0.9 for a straight groove.  相似文献   

6.
Turbulent convective heat transfer and friction factor characteristics of magnetic Fe3O4 nanofluid flowing through a uniformly heated horizontal circular tube with and without twisted tape inserts are estimated experimentally. Experiments are conducted in the particle volume concentration range of 0 < φ < 0.6%,twisted tape inserts of twist ratio in the range of 0 < H/D < 15and Reynolds number range of 3000 < Re < 22000. Heat transfer and friction factor enhancement of 0.6% volume concentration of Fe3O4 nanofluid in a plain tube with twisted tape insert of twist ratio H/D = 5 is 51.88% and 1.231 times compared to water flowing in a plain tube under same Reynolds number. Generalized regression equation is presented for the estimation of Nusselt number and friction factor for both water and Fe3O4 nanofluid in a plain tube and with twisted tape inserts under turbulent flow condition.  相似文献   

7.
Based on the analysis of experimental data the universal methods of calculating convective heat transfer of smooth and finned tube bundles in the crossflow have been developed over the ranges of geometric characteristics covering all practical needs at the Reynolds number Re = 3 × 103…1 × 105.The distinctive feature of the methods proposed is that the generalized similarity equation of convective heat transfer takes into account the dependence of the Reynolds number exponent on tube pitch characteristics in a bundle. This has allowed the mechanism of heat transfer and hydraulic performance in tube bundles to be taken into utmost consideration and the asymptotic character to be given to the generalized dependence. In turn, this has permitted one to show the presence of maximum of heat transfer intensity and also to cover limiting combinations of pitches, at which differences in staggered and in-line arrangements of tubes are leveled, i.e., practically the restrictions on the ranges of tube pitch characteristics of bundles can be removed.  相似文献   

8.
This article presents the condensation heat transfer and flow characteristics of R-134a flowing through corrugated tubes experimentally. The test section is a horizontal counter-flow concentric tube-in-tube heat exchanger 2000 mm in length. A smooth copper tube and corrugated copper tubes having inner diameters of 8.7 mm are used as an inner tube. The outer tube is made from smooth copper tube having an inner diameter of 21.2 mm. The corrugation pitches used in this study are 5.08, 6.35, and 8.46 mm. Similarly, the corrugation depths are 1, 1.25, and 1.5 mm, respectively. The test conditions are performed at saturation temperatures of 40–50 °C, heat fluxes of 5–10 kW/m2, mass fluxes of 200–700 kg/m2 s, and equivalent Reynolds numbers of 30000–120000. The Nusselt number and two-phase friction factor obtained from the corrugated tubes are significantly higher than those obtained from the smooth tube. Finally, new correlations are developed based on the present experimental data for predicting the Nusselt number and two-phase friction factor for corrugated tubes.  相似文献   

9.
《Energy Conversion and Management》2005,46(18-19):2996-3013
Natural convection from uniformly heated helicoidal pipes oriented vertically and horizontally is experimentally studied. Four helicoidal pipes of different parameters are presented. The effects of pitch to pipe diameter ratio, coil diameter to pipe diameter ratio and length to pipe diameter ratio on the average heat transfer coefficient are found. The experiments covered a range of Rayleigh number based on tube diameter from 1.5 × 103 to 1.1 × 105. The results showed that the overall average Nusselt number, Num, increases with the increase in pitch to pipe diameter ratio, coil diameter to pipe diameter ratio and length to pipe diameter ratio for vertical helicoidal pipes. For horizontal helicoidal pipes, the overall average Nusselt number, Num, increases with the increase in pitch to pipe diameter ratio and length to pipe diameter ratio, but it decreases with the increase in coil diameter to pipe diameter ratio. New correlations are presented, and they can be used in HVAC applications.  相似文献   

10.
The effect of enhanced geometry (pore diameter, gap width) is investigated on the pool boiling of R-123/oil mixture for the enhanced tubes having pores with connecting gaps. Tubes having different pore diameters (and corresponding gap widths) are specially made. Significant heat transfer degradation by oil is observed for the present enhanced tubes. At 5% oil concentration, the degradation is 26–49% for Tsat = 4.4 °C. The degradation increases 50–67% for Tsat = 26.7 °C. The heat transfer degradation is significant even with small amount of oil (20–38% degradation at 1% oil concentration for Tsat = 4.4 °C), probably due to the accumulation of oil in sub-tunnels. The pore size (or gap width) has a significant effect on the heat transfer degradation. The maximum degradation is observed for dp = 0.20 mm tube at Tsat = 4.4 °C, and dp = 0.23 mm tube at Tsat = 26.7 °C. The minimum degradation is observed for dp = 0.27 mm tube for both saturation temperatures. It appears that the oil removal is facilitated for the larger pore diameter (along with larger gap) tube. The highest heat transfer coefficient with oil is obtained for dp = 0.23 mm tube, which yielded the highest heat transfer coefficient for pure R-123. The optimum tube significantly (more than 3 times) outperforms the smooth tube even with oil. The heat transfer degradation increases as the heat flux decreases.  相似文献   

11.
The boiling in cross-flow is investigated for coated tubes (low-porosity, flame-sprayed) in this paper. The effect of surface roughness on flow boiling heat transfer for a horizontal tube surface in cross-flow is studied for saturated boiling of water at atmospheric pressure. The parameters varied were for flow velocity up to 3.24 kg/s (G = 258.49 kg/m2 s), heat flux from 12 to 45 kW/m2, surface roughness (Ra) from 0.3296 to 4.731 μm. Nominal enhancement in heat transfer coefficient at higher mass flux may be attributed to the continued nucleation at the uppermost surfaces (in the wake region of the flow) of the rougher tubes thereby increasing the overall heat transfer rate. The flow boiling data was found to best fit the Kutateladze asymptotic equation h = hl[1 + (hnpb/hl)n]1/n with the value of n = 2.258 (which is close to the value of n = 2 suggested by Kutateladze).  相似文献   

12.
The turbulent heat transfer and flow resistance in an enhanced heat transfer tube, the DDIR tube, were studied experimentally and numerically. Water was used as the working fluid with Reynolds numbers between 15,000 and 60,000. The numerical simulations solved the three dimensional Reynolds-averaged Navier–Stokes equations with the standard k-ε model in the commercial CFD code, Fluent. The numerical results agree well with the experimental data, with the largest discrepancy of 10% for the Nusselt numbers and 15% for the friction factors. The heat transfer in the DDIR tube is enhanced 100  120% compared with a plain tube and the pressure drop is increased 170  250%. The heat transfer rate for the same pumping power is enhanced 30  50%. Visualization of the flow field shows that in addition to the front and rear vortices around the ribs, main vortices and induced vortices are also generated by the ribs in the DDIR tube. The rear vortex and the main vortex contribute much to the heat transfer enhancement in the DDIR tubes. Optimum DDIR tube parameters are proposed for heat transfer enhancement at the same pumping power.  相似文献   

13.
Natural convection induced heat transfer has been studied over the outer surface of helically coiled-tube heat exchangers. Several different geometrical configurations (curvature ratio δ ε [0.035, 0.082]) and a wide range of flow parameters (60 <= Ttank <= 90, Tin = 19 and 60 <= Tin <= 90, Ttank = 20, 4000 <= Re <= 45000) have been examined to broaden the validity of the results gained from this research. A fluid-to-fluid boundary condition has been applied in the numerical calculations to create the most realistic flow configurations. Validity of the numerical calculations has been tested by experiments available in the open literature. Calculated results of the inner side heat transfer rate have also been compared to existing empirical formulas and experimental results to test the validity of the numerical computation in an independent way from the outer side validation of common helical tube heat exchangers. Water has been chosen to the working fluid inside and outside of the coiled tube (3 < Pr < 7). Outer side heat transfer rate along the helical tube axis has been investigated to get information about the performance of the heat transport process at different location of the helical tube. It was found that the outer side heat transfer rate is slightly dependent on the inner flow rate of any helical tube in case of increasing temperature differences between the tank working fluid temperature and the coil inlet temperature. A stable thermal boundary layer has been found along the axial direction of the tube.In addition to this the qualitative behavior of the peripherally averaged Nusselt number versus the axial location along the helical tube function is strongly dependent on the direction of the heat flow (from the tube to the storage tank and the reversed direction). Inner side heat transfer rate of helical coils have also been investigated in case of fluid-to-fluid boundary conditions and the calculation results have been compared with different prediction formulas published in the last couples of decades.  相似文献   

14.
An experiment is carried out here to investigate the characteristics of the evaporation heat transfer for refrigerants R-134a and R-407C flowing in horizontal small tubes having the same inside diameter of 0.83 or 2.0 mm. In the experiment for the 2.0-mm tubes, the refrigerant mass flux G is varied from 200 to 400 kg/m2 s, imposed heat flux q from 5 to 15 kW/m2, inlet vapor quality xin from 0.2 to 0.8 and refrigerant saturation temperature Tsat from 5 to 15 °C. While for the 0.83-mm tubes, G is varied from 800 to 1500 kg/m2 s with the other parameters varied in the same ranges as those for Di = 2.0 mm. In the study the effects of the refrigerant vapor quality, mass flux, saturation temperature and imposed heat flux on the measured evaporation heat transfer coefficient hr are examined in detail. The experimental data clearly show that both the R-134a and R-407C evaporation heat transfer coefficients increase almost linearly and significantly with the vapor quality of the refrigerant, except at low mass flux and high heat flux. Besides, the evaporation heat transfer coefficients also increase substantially with the rises in the imposed heat flux, refrigerant mass flux and saturation temperature. At low R-134a mass flux and high imposed heat flux the evaporation heat transfer coefficient in the smaller tubes (Di = 0.83 mm) may decline at increasing vapor quality when the quality is high, due to the partial dryout of the refrigerant flow in the smaller tubes at these conditions. We also note that under the same xin, Tsat, G, q and Di, refrigerant R-407C has a higher hr when compared with that for R-134a. Finally, an empirical correlation for the R-134a and R-407C evaporation heat transfer coefficients in the small tubes is proposed.  相似文献   

15.
An experimental study of surface heat transfer and friction characteristics of a fully developed turbulent air flow in different grooved tubes is reported. Tests were performed for Reynolds number range 10,000–38,000 and for different geometric groove shapes (circular, trapezoidal and rectangular). The ratio of tube length-to-diameter is 33. Among the grooved tubes, heat transfer enhancement is obtained up to 63% for circular groove, 58% for trapezoidal groove and 47% for rectangular groove, in comparison with the smooth tube at the highest Reynolds number (Re = 38,000). Correlations of heat transfer and friction coefficient were obtained for different grooved tubes. In evaluation of thermal performance, it is seen that the grooved tubes are thermodynamically advantageous (Ns, a < 1) up to Re = 30,000 for circular and trapezoidal grooves and up to Re = 28,000 for rectangular grooves. It is observed that there is an optimum value of the entropy generation number at about Re = 17,000 for all investigated grooves.  相似文献   

16.
Experimental investigations of tube side condensation and evaporation in two 3-D enhanced heat transfer (2EHT) tubes were compared to the performance of a smooth surface copper tube. The equivalent outer diameter of all the tubes was 12.7 mm with an inner diameter of 11.5 mm. Both the inner and outer surfaces of the 2EHT tubes are enhanced by longitudinal grooves with a background pattern made up by an array of dimples/embossments. Experimental runs were performed using R410A as the working fluid, over the quality range of 0.2–0.9. For evaporation, the heat transfer coefficient ratio (compares the heat transfer coefficient of the enhanced tube to that of a smooth tube) of the 2EHT tubes is 1.11–1.43 (with an enhanced surface area ratio of 1.03) for mass flux rate that ranges from 80 to 200 kg/m2 s. For condensation, the heat transfer coefficient ratio range is 1.1–1.16 (with an enhanced surface area ratio of 1.03) for mass flux that ranges from 80 to 260 kg/m2 s. Frictional pressure drop values for the 2EHT tubes are very similar to each other. Heat transfer enhancement in the 2EHT tubes is mainly due to the dimples and grooves in the inner surface that create an increased surface area and interfacial turbulence; producing higher heat flux from wall to working fluid, flow separation, and secondary flows. A comparison was performed to evaluate the enhancement effect of the 2EHT tubes using a defined performance factor and this indicates that the 2EHT tubes provides a better heat transfer coefficient under evaporation conditions.  相似文献   

17.
A great number of experimental investigations allowing one to reveal the physical mechanism of processes responsible for their thermal and hydraulic performance are carried out in attempt to solve problems of updating constructions and methods of thermal design of heating surfaces of transversely finned tubes widespread in power engineering. Results of flow visualization and investigation of pressure fields and local heat transfer at the fin surface over the Reynolds number range Re = (1.0 ? 6.6) · 104 are presented for the case of a wide variation of geometric characteristics of finned tubes and parameters of their arrangement in a bundle. Regularities substantially changing the existing concept of transfer processes in the interfin space and in the wake behind a finned tube are revealed. It is found that the flow behavior and the distribution of local heat transfer coefficients over the fin surface change significantly at the fin height-to-finned tube diameter h/d approximately equal to 0.4. The results obtained are generalized in the form of the patterns of flow and heat transfer at the finned tube surface, including seven characteristic regions and four types of flow separation.  相似文献   

18.
The effect of the flow geometry parameters on transient forced convection heat transfer for turbulent flow in a circular tube with baffle inserts has been investigated. The characteristic parameters of the tubes are pitch to tube inlet diameter ratio H/D = 1, 2 and 3, baffle orientation angle β = 45°, 90° and 180°. Air, Prandtl number of which is 0.71, was used as working fluid, while stainless steel was considered as pipe and baffle material. During the experiments, different geometrical parameters such as the baffle spacing H and the baffle orientation angle β were varied. Totally, nine types of baffle inserted tube were used. The general empirical equations of time averaged Nusselt number and time averaged pressure drop were derived as a function of Reynolds number corresponding to the baffle geometry parameters of pitch to diameter ratio H/D, baffle orientation angle β, ratio of smooth to baffled cross-section area So/Sa and ratio of tube length to baffle spacing L/H were derived for transient flow conditions. The proposed empirical correlations were considered to be applicable within the range of Reynolds number 3000  Re  20,000 for the case of constant heat flux.  相似文献   

19.
Numerical and experimental analyses were carried out to study thermal–hydraulic characteristics of air flow inside a circular tube with different tube inserts. Three kinds of tube inserts, including longitudinal strip inserts (both with and without holes) and twisted-tape inserts with three different twisted angles (α = 15.3°, 24.4° and 34.3°) have been investigated for different inlet frontal velocity ranging from 3 to 18 m/s. Numerical simulation was performed by a 3D turbulence analysis of the heat transfer and fluid flow. Conjugate convective heat transfer in the flow field and heat conduction in the tube inserts are considered also. The experiments were conducted in a shell and tube exchanger with overall counterflow arrangement. The working fluid in the tube side was cold air, while the hot Dowtherm fluid was on the shell side. To obtain the heat transfer characteristics of the test section from the experimental data, the ε-NTU (effectiveness-number of transfer unit) method is applied to determine the overall conductance (UA product) in the analysis.It was found that the heat transfer coefficient and the pressure drop in the tubes with the longitudinal strip inserts (without hole) were 7–16% and 100–170% greater than those of plain tubes without inserts. When the longitudinal strip inserts with holes were used, the heat transfer coefficient and the pressure drop were 13–28% and 140–220%, respectively, higher than those of plain tubes. The heat transfer coefficient and the pressure drop of the tubes with twisted-tape inserts were 13–61% and 150–370%, respectively, higher than those of plain tubes. Furthermore, it was found that the reduction ratio in the heat transfer area of the tube of approximately 18–28% may be obtained if the twisted-tape tube inserts are used.  相似文献   

20.
The current paper presents experimental investigation of nucleate pool boiling of R-134a and R-123 on enhanced and smooth tubes. The enhanced tubes used were TBIIHP and TBIILP for R-134a and R-123, respectively. Pool boiling data were taken for smooth and enhanced tubes in a single tube test section. Data were taken at a saturation temperature of 4.44 °C. Each test tube had an outside diameter of 19.05 mm and a length of 1 m. The test section was water heated with an insert in the water passage. The insert allowed measurement of local water temperatures down the length of the test tube. Utilizing this instrumentation, local heat transfer coefficients were determined at five locations along the test tube. The heat flux range was 2.5–157.5 kW/m2 for the TBIIHP tube and 3.1–73.2 kW/m2 for the TBIILP tube. The resulting heat transfer coefficient range was 4146–23255 W/m2. °C and 5331–25950 W/m2. °C for both tubes, respectively. For smooth tube testing, the heat flux ranges were 7.3–130.7 kW/m2 and 7.5–60.7 kW/m2 for R-134a and R-123, respectively; with resulting heat transfer coefficient ranges of 1798.9–11,379 W/m2. °C and 535.4–3181.8 W/m2. °C. The study provided one of the widest heat flux ranges ever examined for these types of tubes and showed significant structure to the pool boiling curve that had not been traditionally observed. Additionally, this paper presented an investigation of enhanced tubes pool boiling models.  相似文献   

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