Tube-side Heat Transfer Research Articles

TUBE-SIDE TWO-PHASE HEAT TRANSFER COEFFICIENTS OF REFRIGERANT HFC-134a FLOWING THROUGH A FIN-AND-TUBE EVAPORATOR

In the present study, the two-phase heat transfer coefficient characteristics of HFC-134a, evaporating inside a plate fin-and-tube evaporator with plain fin geometry, are experimentally investigated. The experimental apparatus consists essentially of a well modified vapour compression refrigeration system. Evaporator and condenser are made from aluminium plate finned, copper tube. In addition to the loop components, a full set of instruments for measuring temperature, pressure and flow rate of the refrigerant and the air are installed at the important points in the circuit. The test runs are done at average saturated refrigerant temperatures ranging between 4.0 and 9.0 °C for evaporation and ranging between 39.0 and 46.0 ° C for condensation. The refrigerant mass flow rates are between 7.6 x 10 -3 and 9.0 x × 10 -3 kg/s. The volumetric flow rate of air passing through the evaporator ranges between 0.25 and 0.5 m 3 /s and between 0.7 and 1.25 m 3 /s for the condenser. The average amount of heat exchanged between the refrigerant-side and air-side is between 1270 and 1770 W for the evaporator and between 1740 and 2200 W for the condenser. The experimental data obtained from the present study together with various relevant parameters from literature are used to determine the tube-side evaporation heat transfer coefficients. The results from the present study are compared with those calculated from correlations reported in literature. Moreover, a new correlation based only on the data gathered during this work is proposed for practical application.

Data reduction for air-side performance of fin-and-tube heat exchangers

The present study focuses on the data reduction method to obtain the air-side performance of fin-and-tube heat exchangers. The data reduction methodology for air-side heat transfer coefficients in the literature is not based on a consistent approach. This paper recommends standard procedures for dry surface heat transfer in finned-tube heat exchangers having water on the tube-side. Inconsistencies addressed include the ε-NTU relationships, calculation of the tube-side heat transfer coefficient, calculation of fin efficiency, and whether entrance and exit loss should be included in the reduction of friction factors. Use of the recommended standardized methodology will provide more meaningful data for use in the development of correlations, or for performance comparison purposes.

THREE DIMENSIONAL NUMERICAL SIMULATION OF SHELL-AND-TUBE HEAT EXCHANGERS. PART I: FOUNDATION AND FLUID MECHANICS

A three-dimensional, colocated, fully implicit, control volume based calculation procedure, HEATX[1], has been used to simulate fluid flow and heal transfer in shell-and-tube heat exchangers. The three-dimensional numerical model uses the distributed resistance method along with volumetric porosities and surface permeabilities to model the tubes in the heat exchanger. Turbulence effects are modeled using a modified k-ϵ model with additional source terms for turbulence generation and dissipation by tubes. Shell and baffle walk are modeled using the wall function approach. Tubes and baffles are modeled using volumetric porosities and surface permeabilities. Baffle-shell and baffle-tube leakages are modeled using a Bernoulli type formulation. Specialized geometry generators compute baffle, nozzle, and tube region porosities and permeabilities. This article presents the foundation and fluid mechanics of the problem. A subsequent article will discuss modeling of shell-side and tube-side heat transfer. The three-dimensional numerical model is validated by comparison of computed pressure drops with the experiments conducted at Argonne National Labs [2] in E shell type heat exchangers. The effect of baffle cut and baffle spacing on the pressure drop is studied. Good agreement is obtained between the computed results and the experiments.

THREE-DIMENSIONAL NUMERICAL SIMULATION OF SHELL-AND-TUBE HEAT EXCHANGERS. PART II: HEAT TRANSFER

A three-dimensional, allocated, fully implicit control volume based calculation procedure HEATX [1] has been developed over the past 3 years to simulate flow and heat transfer in sheU-and-tube heat exchangers. The three-dimensional model uses the distributed resistance concept of Patankar and Spalding [2], in conjunction with surface permeabilities and volumetric porosities to model the tubes in the heat exchanger. Part I of this article describes the details of the distributed resistance formulation, leakage modeling, geometry modeling, and the turbulence model. Details of the shell-side and tube-side heat transfer are given in this second part. The tube-side temperature field is computed by solving an enthalpy equation for the tube-side fluid. Coupling between the shell-side and the tube-side equations is described, and numerical results are compared with the Delaware project experimental data. We have made use of the symmetry of the heat exchanger to speed up our calculations. Good agreement was obtained between our three-dimensional numerical simulations and experimental results for overall pressure drop and temperature differences. Computed overall pressure drops and temperature differences were within 15% of experimental results.

Turbulent Flow in Integrally Enhanced Tubes, Part 1: Comprehensive Review and Database Development

Abstract Thermal performance improvements through passive and active enhancement methods have been studied intensely for over three decades. Heat exchangers with single-phase lube-side flow are one of the important applications where passive surface-modification methods are popularly employed. Ribs, indentations, spiral flutes, and coil inserts are some common surface modification techniques that are relatively easy to manufacture, are cost effective for many applications, and can be used conveniently for retubing existing heat exchangers. This article describes a comprehensive database that was developed to provide easy access to existing single-phase tube-side heat transfer and pressure drop data for different enhanced tube designs, in a consistent and useful manner. The database contains four files: (1) friction factor data, (2) heat transfer data, (3) tube characteristics, and (4) features of the experiments used by each data source, such as fluid type, boundary conditions, tube material, and tube type. The friction factor database contains nearly 5,000 data points, and the heat transfer database contains more than 4,700 data points. The data encompass more than 360 different tube geometries from over 35 investigations. The information can be used by practicing engineers for validatating and /or developing prediction methods, or to compare or estimate the performance of a specific tube design.

Radiative enhancement of tube-side heat transfer

The potential of augmenting film coefficient by uniformly dispersing thin metallic/ceramic filaments oriented longitudinally along a tube is investigated. The purpose of the rigidly held filaments is to create a participating medium from a gas otherwise transparent to thermal radiation. The filaments absorb the thermal radiation emitted by the tube and transfer the heat convectively to the flowing gas. Wave theory shows that optical thickness > 10 can be achieved with 50 μm SiC filaments at 300 cm −2 number density in a 2.54 cm diameter tube. Solution of the radiation transport equation indicates that the radiative film coefficients are a function of filament material, diameter and number density, and gas and surface temperatures.

An Experimental Investigation of the Augmentation of Tube-Side Heat Transfer in a Crossflow Heat Exchanger by Means of Strip-Type Inserts

The results of an experimental investigation of the augmentation of heat transfer in a crossflow heat exchanger by means of strip-type inserts are presented and discussed. Three kinds of strip type inserts—longitudinal strip (LS), crossed-strip (CS), and regularly interrupted strip (RIS)—are studied. Correlations for friction factor and Nusselt number are also developed. It is shown that on the basis of both constant pumping power and constant heat duty, LS-inserts perform better than CS- and RIS-inserts for tube-side Reynolds numbers above 19,000. At tube-side Reynolds numbers below 12,000, CS-inserts perform better than LS- and RIS-inserts. Furthermore, flow visualization was made for shell-side hot air and cold air flow in the plenum to advance the qualitative understanding of the physical phenomena of this type of crossflow heat exchanger.

Development of a Method to Evaluate the Design Performance of a Feedwater Heater With a Short Drain Cooler

A method is developed to determine the shell and tube side heat transfer performance of a feedwater heater with a short drain cooler. The desuperheating, condensing, and drain cooling zones are discussed and analyzed by deriving a modified version of the Delaware Method of Shell-Side Design.

ENHANCED TUBES FOR STEAM CONDENSERS

Abstract This article provides data on enhanced tubes developed especially for application to steam condensers. Data are provided for different types of tube-side (cooling water) and shell-side (steam) enhancements. The first test series involved measuring the condensing coefficient at a 327 K saturation temperature. Average enhancement levels (defined as the ratio of the condensing coefficient of an enhanced tube to that of a plain tube at the same vapor-to-wall temperature difference) of 34%, 75%, and 180% were obtained for titanium, copper-nickel, and copper, respectively. The second test series determined the overall heat transfer coefficient of several tubes having both internal and external enhancement geometries. Maximum enhancement levels of the overall heat transfer coefficient were 16%, 37%, and 64% (at the same mass flow rate for both the plain and enhanced tube) for titanium, stainless steel, and copper-nickel, respectively. A third data set was obtained for the tube-side heat transfer and pressure drop characteristics of several internal enhancements. Using specific combinations of the identified internal and external geometries, other potential enhanced tubes were defined, and a comparison of the overall heat transfer coefficient was made.

Effects of working fluid, tubeside enhancement and bundle depth on the optimized fin geometry of a horizontal condenser tube

A theoretical study has been made to optimize the fin geometry of a horizontal finned tube which is to be used for condensers that handle the vapor load of a liquid phase change cooling module. Systematic numerical calculations of the vapor to coolant heat transfer have been performed for parametric values of fin height, fin spacing, vertical bundle depth and tubeside heat transfer coefficient. Three dielectric fluids (R-113, FC-72, and FC-87) at atmospheric pressure were selected as the working fluids. For a single tube with optimized fin geometry, the average heat flux increased in the order of FC-87, R-113 and FC-72. Both the optimum fin height and optimum fin spacing increased with increasing vertical bundle depth.

Assessment study of longitudinal rectangular plate inserts as tubeside heat transfer augmentative devices

A number of longitudinal rectangular plate inserts used as tubeside heat transfer augmentative devices have been numerically investigated and assessed in the present study. The tube is of the axially uniform heat flux type with a peripherally constant wall temperature and an adiabatic rectangular plate insert. The flow is laminar, thermally developed and of steady state. The effects of the aspect ratio of the rectangular plate, the radius ratio of the circumscribed circle of the rectangular plate to the tube, and the eccentric installation on the net effect between heat transfer augmentation and pressure drop increase are determined.

Augmentation of highly viscous laminar heat transfer inside tubes with constant wall temperature

An experimental investigation of enhanced tubeside flow and heat transfer under laminar flow conditions is reported. The three test sections in this study included a plain tube, an internally finned tube, and a tube with a twisted-tape insert, all with a nominal diameter of 25.4 mm and a length of 2.44 m. A highly viscous Newtonian liquid, Polybutene 20, was used as the test fluid. The heat transfer results were obtained in a horizontal test section with nearly constant wall temperature. Isothermal pressure drop, nonisothermal pressure drop, and heat transfer data were obtained over a wide range of the parameters: Reynolds number, 15.1–575; Prandtl number, 1260–8130; Graetz number, 868–6570; and bulk-to-wall viscosity ratio, 0.00464–42.8. The effects of free, or natural, convection were negligible in this study. All parameters are evaluated at the arithmetic mean bulk temperature and are based on the inside envelope diameter. Isothermal friction factors for the internally finned tube and the tube with the twisted-tape insert are 1.7 and 3.4 times those of the plain tube for the range of Reynolds numbers tested, with the nonisothermal friction factors showing similar increases. Augmented heat transfer coefficients were above the plain tube values for both heating and cooling. The greatest improvement for heating was for the internally finned tube where Nusselt numbers ranged from 3 to 4 times the plain tube values at corresponding Graetz numbers. For cooling, the twisted-tape Nusselt numbers were 1.5–2.25 times the corresponding plain tube values. Tabulated values and empirical correlations are presented for all of the data obtained.

Experimental Investigation of the Augmentation of Forced Convection Heat Transfer in a Circular Tube Using Spiral Spring Inserts

Results of the experimental investigation of a class of spiral spring coil used as a tube side heat transfer augmentative device for a single phase cooling mode operation are presented. SAE 10 engine oil flowing inside the tube is cooled by water flowing outside the tube. This spiral spring insert is inexpensive but it can increase the tube side heat transfer coefficient significantly. Thus its use as an augmentative device is effective. Application of this device in the design of oil coolers is discussed.

Experimental studies on local evaporating heat transfer of internal spiral-grooved tube.

To improve tube-side heat transfer coefficients of air-cooled heat exchangers in air-conditioners, internal spiral-grooved tubes have been put to use in place of plain tube. In this report, local evaporating heat transfer coefficients have been measured for internal spiral-grooved tube with refrigerant R22. Experimental results show that the local heat transfer coefficients are estimated by and the reason why there is little dependence on quality is due to the effects of grooves. Further discussions will be developed in following paper.

Tube-Side Heat Transfer and Pressure Drop for Tubes Having Helical Internal Ridging with Turbulent/Transitional Flow of Single-Phase Fluid. Part 2. Multiple-Helix Ridging

Tube-Side Heat Transfer and Pressure Drop for Tubes Having Helical Internal Ridging with Turbulent/Transitional Flow of Single-Phase Fluid. Part 2. Multiple-Helix Ridging

Tube-Side Heat Transfer and Pressure Drop for Tubes Having Helical Internal Ridging with Turbulent/Transitional Flow of Single-Phase Fluid. Part 1. Single-Helix Ridging

An experimental investigation was conducted of the tube-side behavior of various internal configurations with turbulent/transitional flow of water in the Reynolds number range 10,000-120,000 and Prandtl number range 2-11. By using a heat-momentum transfer analogy, correlations were obtained that enable practical designs with standard products or optimization of tube geometry for specific conditions.

Heat transfer and pressure drop of corrugated tubes

Effects of the shapes of corrugated tubes hd.ch affect heat transfer and pressure drop characteristics were experimentally studied. All heat transfer experiments were conducted with steam at atomospheric pressure condensing on the outside of a horizontally mounted 90–10 cupronickel tube. Tube side pressure drop was measured under isothermal cold flow condition. The results show that tube aide heat transfer coefficients decreased with the increose in pitch and reached a maximum at the ratio 0.05 of indentation depth to inside tube diameter and that the tube side coefficients of two start corrugated tubes were slightly smaller than those of single start tubes. Friction factors were expressed by the equation including pitch and indentation depth. From these results, the relation between tube side heat transfer coefficients and pressure drop per unit length was presented.