Rough Tubes Research Articles

Numerical and experimental investigation of enhancement of heat transfer in dimpled rib heat exchanger tube

In this study, numerical and experimental investigation has been carried out for a range of system and operating parameters in order to analyse the effect of dimpled rib on heat and fluid flow behaviours in heat exchanger tube. Tube has, stream wise spacing (x/d d ) range of 15–35, span wise spacing (y/d d ) range of 15–35, ratio of dimpled depth to print diameter (e/d d ) of 1.0 and Reynolds number (Re n ) ranges from 4000 to 28,000. Simulations were carried out to obtain heat and fluid flow behaviour of smooth and rough tube, using commercial CFD software, ANSYS 16.0 (Fluent). Renormalization k − e model was employed to assess the influence of dimpled on turbulent flow and velocity field. Simulation results show that, the enhancement of 3.18 times in heat transfer and 2.87 times enhancement in thermal hydraulic performance as a function of stream wise direction (x/d d ) of 15 and span wise direction (y/d d ) of 15 respectively. Comparison between numerical and experimental simulation results showed that good agreement as the data fell within ±10% error band.

DETERMINATION OF THE OPTIMAL SPEED OF THE MANDREL IN THE CONTINUOUS REELING MILL USING NUMERICAL SIMULATION

A proper numerical model of expanding process on a three roll continuous reeling mill was created and number of numerical experiments of the process of tubes rolling were performed. Based on the analysis of the stress-strain state of a rough tubes deformation zone, optimal speed rates of the mandrel providing the decrease in mill rolls and deterioration of mandrel which direct influences on the quality of rough tubes were determined.

Numerical analysis of thermal hydraulic performance of [formula omitted]–[formula omitted] nanofluid flowing through a protrusion obstacles square mini channel

In this study, numerical investigation has been carried out for a range of system and operating parameters in order to analyze the effect of protrusion obstacles on heat and fluid flow characteristics of nanopartical Al2O3 with concentration 4.0% and diameter 30 nm in square mini channel. The square mini channel has, stream wise spacing (Xs/dp) range of 1.4–2.6, span wise spacing (Ys/dp) range of 1.4–2.6, ratio of protrusion height to print diameter (ep/dp) of 1.67 and Reynolds number (Ren) ranges from 4000 to 18,000. Simulations were carried out to obtain heat and fluid flow behaviour of smooth and rough tube, using commercial CFD software, ANSYS 16.0 (Fluent). Renormalization k−ε model was employed to assess the influence of protrusion on turbulent flow and velocity field. Simulation results show that, the enhancement of 3.73 times in heat transfer and 4.25 times enhancement in pressure drop as a function of Xs/dp and Ys/dp of 1.8 respectively.

Optimization of the Friction Factor and Frictional Pressure Drop of R22 and R290

Today, the air-conditioning and refrigeration industry is still searching for the environmentally friendly refrigerants that could replace the hazardous coolants that are ozone depleting – refrigerants that behave similarly, if not better than the present ones. The present study includes optimization of the frictional pressure drop of R22 and R290 using genetic algorithm. Parametric analysis have been conducted in order to determine the difference in the case of the use of genetic algorithms. The outcomes are compared against that of the measured pressure drop, the latter obtained from a horizontal 7.6 mm channel with a length of 1.07 meter. Three equations have been used for calculating the Darcy friction factor and two-phase flow pressure drop for both laminar and turbulent flow regimes in smooth and rough tubes. The effects of the different correlations for the friction factor and the pressure drop utilized are demonstrated. The results illustrate that the differences between values of the Darcy friction factor are very small for the two refrigerants, and the frictional pressure of R-290 higher than R-22. Smaller channel induced much higher frictional pressure drop too.

Heat transfer and fluid flow characteristics of heat exchanger tube with multiple twisted tapes and solid rings inserts

Heat transfer and fluid flow characteristic of a roughened tube is investigated for constant heat flux. The inserts were employed for the swirl flow generation in the test section through the tube. Pitch ratios (l/D)=1 and 2 of the solid ring tubular (SRT) and twist ratios (y/W)=2, 3 and 4 of the twisted tapes (TT) are introduced into the core flow. The Reynolds number covered in the range of 6300–22500 for co-twist arrangements for the turbulent flow of air in the tested heat exchanger tube. The study reveals that the Nusselt number, friction factor and thermal performance factor of the integrated SRT and TT are in the range of 107–293, 0.93–0.99 and 1.46–1.61, respectively. The highest thermal performance factor in the order of 1.61 over smooth tube heat exchanger was obtained for the integrated device consisting of the SRT with l/D=1.0 and y/W=2. The quadruple co-twisted tape insert provides the maximum Nusselt number, friction factor, thermal performance factor. The correlations for the Nusselt number, friction factor and thermal performance factor of the tubes with combined devices were also developed in terms of flow and geometrical parameters.

FEM SIMULATION OF A NEW METHOD OF LENGTHWISE PIPE ROLLING ON A STUB MANDREL WITH STRETCH

The improvement of the pipe production technology at the lengthwise rolling mill was suggested. The investigations were carried out on the basis of the lengthwise rolling mill “tandem” installed at Open Joint-Stock Company «Sinarsky Pipe Works». There is an aim of mastering the whole gauge production of continuous casting on the existing equipment to reduce the production costs with the exclusion of diameter 120 mm rolled workpiece. The elongation ratio increase on the “tandem” lengthwise rolling mill is necessary to develop the continuous casting. At present time the elongation ratio on the lengthwise rolling mill no. 1 is between 1.16 and 1.5 and elongation ratio on lengthwise rolling mill no. 2 is between 1.07 and 1.15. The use of the increased elongation ratios on automatic mill “tandem” leads to increase of strap sizes and as result to increase of the amount of waste at a “guide mark” defect. The way to reduce the probability of the formation of a “guide mark” defect due to the kinematic tension during rolling on a stub mandrel with rough tube preovalisation was investigated. The infl uence of the kinematic tension on the forming tube in the groove taper at a lengthwise rolling with rough tube preovalisation was studied. The authors described the possibility of increasing the elongation ratio on the automatic mill “tandem”.

Study of Turbulent Convective Heat Transfer Enhancement by Al<sub>2</sub>O<sub>3</sub>-Water Nanofluid through a Rough Circular Tube

Nanotechnology is a novel approach in thermal engineering science to enhance the overall thermal performance of compact heat exchangers by the homogeneous dispersion of solid nanoparticles of higher thermal conductivity in conventional base fluid like water, oil, ethylene glycol etc. The heat transfer rate is substantially intensified by the addition of nanosized solid particles which provide superior thermo-physical properties in comparison with base fluid. In the present study, a numerical simulation is performed to investigate the turbulent convective heat transfer characteristics of Al2O3-water nanofluid of volume fractions (1%, 3% and 5%) through a rough circular tube subjected to constant heat flux for a range of Reynolds number 10,000 to 30,000. The finite volume method is employed for solving the governing equations and k-ω SST turbulent model for single phase analysis is considered. At a Reynolds number of 25000, application of nanofluid combined with rough tube enhances the Nusselt number by 13.10%, 21.86% and 63.03% in case of relative roughness of the wall of 0.001, 0.005 and 0.01 respectively.

Numerical investigation on heat transfer of supercritical water in a roughened tube

ABSTRACTThe turbulent mixed convection heat transfer of supercritical water flowing in a vertical tube roughened by V-shaped grooves has been numerically investigated in this paper. The turbulent supercritical water flow characteristics within different grooves are obtained using a validated low-Reynolds number κ-ε turbulence model. The effects of groove angle, groove depth, groove pitch-to-depth ratio, and thermophysical properties on turbulent flow and heat transfer of supercritical water are discussed. The results show that a groove angle γ = 120° presents the best heat transfer performance among the three groove angles. The lower groove depth and higher groove pitch-to-depth ratio suppress the enhancement of heat transfer. Heat transfer performance is significantly decreased due to the strong buoyancy force at Tb = 650.6 K, and heat transfer deterioration occurs in the roughened tube with γ = 120°, e = 0.5 mm, and p/e = 8 in the present simulation. The results also show that the rapid variation in the supercritical water property in the region near the pseudo-critical temperature results in a significant enhancement of heat transfer performance.

New friction factor equations developed for turbulent flows in rough helical tubes

Helical tubes are widely used and there lacks friction factor equations for turbulent flows in rough helical tubes. Turbulent flows in rough helical tubes were investigated theoretically in this paper. Friction factor equations for transitionally and fully rough regime turbulent flows in rough helical tubes were derived based on the logarithmic velocity distribution law. The parameters of the equations were obtained by regression analysis of experimental data of Clancy (1949) and verified by experimental data of McElligott (1948). The characteristics of the friction factors in rough helical tubes were discussed based on the equations. Friction factors of helical tubes are influenced by Reynolds numbers, relative roughnesses and curvature ratios. The friction factors of rough helical tubes can also be divided into a transitionally rough regime and a fully rough regime according to the roughness height and the Reynolds number like that for straight tubes. Roughnesses have greater effects on the transition from the transitionally rough regime to the fully rough regime than that of curvature ratios. Roughness effects on friction factors increase with the increasing of the curvature ratio and the Reynolds number.

Influence of thermal shock on fouling of smooth, rough and finned tubes

Deposits of salts and extraneous materials occur during water desalination by heating, forming an insulating layer, which is known as fouling. Fouling reduces the rate of heat transfer between the heating medium and the water, subsequently decreases the desalination efficiency gradually and sometimes can lead to operation failure. The thermal shock is a fouling mitigation technique in water desalination units, which could be due to sudden decrease or increase in the heat transfer process. The objective of this research is to study the removal of fouling layers by the thermal shock in case of (i) smooth tubes, (ii) rough tubes and (iii) finned-rough tubes. It has been found that it is possible to remove the fouling layer by the thermal shock technique in case of smooth and rough tubes, but not in case of finned tubes. The thermal shock blows off the fouling layer in case of plain tubes, and the fouling process resumes after the thermal shock without any delays, i.e. no more induction period. Fins assist the sticking of crystals to the heat exchanger tubes and prevent its removal by thermal shock. Fins act as a heat sink during the thermal shock, which reduces the dissipation of heat to the surrounding water, i.e. cooling of the tubes, and prevent cracking of the fouling layer.

Finite-Element Simulation of the Process of Lengthwise Pipe Rolling on a Stub Mandrel

The improvement of the technology of pipe production on a lengthwise rolling mill is proposed. The investigations are carried out on the basis of a lengthwise rolling mill installed at the “Sinarsky Pipe Works” Open Joint-Stock Company. The aim is to master the whole range of production from continuous castings on the existing equipment, as well as to exclude 120 mm diameter rolled workpiece from the production in order to reduce production costs. An increase in the elongation ratio on a lengthwise rolling mill is necessary to master a continuous casting. Rough tube forming on a stub mandrel in a lengthwise rolling mill with rough tube preovalisation has been investigated with the application of the finite-element method. The behavior of the dimensionless parameters characterizing the deformation of the metal in the groove taper as dependent on the rough tube elongation ratio has been determined. The bite begins in the upper part of the groove when the oval rough tube is fed to the lengthwise rolling mill groove. Rough tube preovalisation provides uniform groove filling during lengthwise rolling on a tapered stub mandrel. Besides, it provides a decrease in the intensity of rough tube wall thickening in the groove taper during lengthwise rolling. This enables one to avoid the formation of a “guide mark” defect on the inner surface. The ratio of the longer axis of the rough tube oval cross section to the minor axis must range between 1.15 and 1.25. The rough tube preovalisation before lengthwise rolling provides an opportunity to increase elongation ratio at the lengthwise rolling mill from 1.45 to 2.00. Keywords: A tube rolling-off; a stub mandrel; an automatic mill “tandem”; a finite-element simulation; a coefficient of rough tube ovalisation.

The Effect of Reynolds Number on the Thermal and Hydrodynamic Characteristics of Turbulence Flow of the Nanofluid in the Heat Exchanger

Abstract. Reynolds number is one of the most important parameters in investigation of heat transfer in double tube heat exchangers. In this paper, the effect of this parameter has been investigated on the convective heat transfer coefficient and surface friction coefficient of the wall. Turbulent forced convection heat transfer of nanofluid flow of Al 2 O 3 /water in a double tube heat exchanger with rough tubes in the annular portion was numerically studied. The finite volume method and the second-order upstream difference scheme were used for the discretization of the governing equations. The volume fraction and mean diameter of nanoparticles were assumed to be 4% and 32 nm, respectively. After reviewing the results, it was observed that the convective heat transfer coefficient in the outer and inner walls of heat exchanger increases with the increase of the Reynolds number. The wall surface friction coefficient, which decreases by increasing Reynolds number, is another examined parameter.

Rolling hot-deformed pipe on a stub mandrel

A new method of rolling hot-deformed pipe on an automatic tandem mill consisting of two mills for longitudinal pipe rolling on a stub mandrel is proposed. In this method, the rough tube becomes oval immediately ahead of the rollers in the longitudinal-rolling mills. That ensures uniform filling of the groove and more uniform wall thickness of the pipe. The use of an oval rough tube increases the reduction on the first longitudinal-rolling mill from an elongation ratio of 1.5 to 2 and correspondingly decreases the reduction in the piercing mill. As a result, there is less risk of rolling skin on the outer surface of the pipe.

EFFECT OF TUBE DIAMETER AND SURFACE ROUGHNESS ON FLUID FLOW FRICTION FACTOR

Experiments have been performed to investigate the effect of channel roughness and diameter on fluid friction. Three different diameters and roughness of tubes were used to examine the friction factor. The first tube made of stainless steel with an inner diameter of 1.14 mm was investigated at Brunel University, whilst the others made of PVC with diameters of 17 mm and 15.5 mm rough were tested at Mataram University. The stainless steel was equipped with a 200 mm calming section and smooth one. The 15.5 mm diameter tube was coated internally with sand that had an average grain size of 0.5 mm so that the tube had a relative roughness of 0.032. The last tube with a diameter of 17 mm was smooth as explained in the H408 Fluid Friction Experimental Apparatus manual. The results indicate that the flow in the stainless steel tube still obeys the theory and in the 17 mm tube shows a deviation in friction factor with the theory. However, this was due to no calming section installed in the test rig. Flow in the rough tube (15.5 mm diameter) demonstrates that the Reynolds number does not affect the friction factor in turbulent regimes and the experimental friction factors were reasonably in a good agreement with the theory or Moody diagram. Hence, the effect of decreasing in diameter of channels on friction factor is insignificant.

Structural annealing of carbon coated aligned multi-walled carbon nanotube sheets

Sheets of aligned multi-walled carbon nanotubes (AMWCNTs) were used to study the structural annealing of pyrolytic carbon (PyC) coatings with various thicknesses on MWCNTs. PyC was deposited using chemical vapor infiltration and the thickness was controlled via the infiltration time. Structural annealing of the PyC coated AMWCNT (AMWCNT/C) sheets at 2150°C provided different results for different thickness coatings. Transmission electron microscopy images showed that the carbon deposited from acetylene formed laminar PyC coatings, resembling rough tube walls, on the CNT surfaces. Following the high temperature heat treatment, coatings from short PyC deposition times changed their structure, resulting in radial growth of the MWCNTs. Raman and X-ray diffraction measurements also revealed that the radially grown MWCNTs had graphitic quality very close to pristine nanotubes after annealing. Electrical conductivity of AMWCNT/C sheets after high temperature heat treatment was twice that of pristine AMWCNT sheets. The focus of this study was to determine the PyC coating thickness at which a rough PyC coating would no longer change its structure into new CNT walls. The samples treated longer than 30min had much more disordered PyC deposited on the surface and the additional material did not form additional tube walls after thermal annealing.

Detonation limits in rough walled tubes

The present paper reports the results of a study of detonation limits in rough tubes. Detonation velocity is measured by photodiodes and ionization probes spaced at 10cm intervals along the length of the tube. Short lengths of smoked foils inserted into the core of the rough tube is used to register the structure of the detonation wave. Pressure transducers are also used to obtain the pressure profile. The results indicate that in rough tubes, the detonation velocity is generally much lower than the corresponding values for smooth tubes. The velocity decreases slowly at first and then more rapidly as the limit is approached. The velocity variation is generally continuous and at the limits, the failure velocity is of the order of about 0.4 VCJ for all cases. The detonation limits in rough tubes are found to be wider than for a smooth tube. This indicates that the turbulence generated by the wall roughness facilitates the propagation of the detonation and extends the limits. Smoked foil records show that in the core of the rough tube the detonation front has a cellular structure corresponding to the usual cellular structure due to instability of the detonation. Thus the intrinsic unstable cellular structure is quite robust and retains its global characteristics in spite of the large perturbations generated by the rough wall. The detonation in the core of the rough tube goes from multi-headed to single headed as the limit is approached. Past the single headed spin, the low velocity detonation has no cellular structure but consists of interacting weak transverse waves from the rough wall. The averaged pressure of the low velocity detonation front corresponds to about the constant volume explosion pressure, in accord with the velocity of the low velocity detonation.

Pool boiling of nanofluids on rough and porous coated tubes: experimental and correlation

Abstract The paper deals with pool boiling of water-Al2O3 and water- Cu nanofluids on rough and porous coated horizontal tubes. Commercially available stainless steel tubes having 10 mm outside diameter and 0.6 mm wall thickness were used to fabricate the test heater. The tube surface was roughed with emery paper 360 or polished with abrasive compound. Aluminium porous coatings of 0.15 mm thick with porosity of about 40% were produced by plasma spraying. The experiments were conducted under different absolute operating pressures, i.e., 200, 100, and 10 kPa. Nanoparticles were tested at the concentration of 0.01, 0.1, and 1% by weight. Ultrasonic vibration was used in order to stabilize the dispersion of the nanoparticles. It was observed that independent of operating pressure and roughness of the stainless steel tubes addition of even small amount of nanoparticles augments heat transfer in comparison to boiling of distilled water. Contrary to rough tubes boiling heat transfer coefficient of tested nanofluids on porous coated tubes was lower compared to that for distilled water while boiling on porous coated tubes. A correlation equation for prediction of the average heat transfer coefficient during boiling of nanofluids on smooth, rough and porous coated tubes is proposed. The correlation includes all tested variables in dimensionless form and is valid for low heat flux, i.e., below 100 kW/m2.

An Experimental Investigation on Friction Characteristics and Heat Transfer of Air and CO2 Flow in Microtubes With Structured Surface Roughness

Experiments were conducted to investigate roughness effects on flow characteristics and heat transfer of air and CO2 flow in four circular micr-tubes of approximately 1 mm inner diameter. The tubes were made by electrodepositing nickel on an aluminum sacrificial substrate. The desired roughness structures were machined or etched on the substrate before depositing nickel to generate a replica of the aluminum substrate on the internal surface of the nickel tubes. Four different surface roughness features were generated: (i) uniform roughness of 3.8 m, (ii) uniform roughness of 1.8 m, (iii) internal grooves 45 m deep and 218 m wide, and (iv) helical grooves 96 m deep with 1.9 mm pitch. Friction factor and Nusselt number data for the smooth tube are in good agreement with the conventional correlations in both the laminar and the turbulent flow regimes. In the rough tubes, the friction factors are significantly higher than that of the smooth tube. Heat transfer coefficients in the laminar flow regime for both smooth and rough tube agree well with the conventional correlation. However, in the turbulent flow regime, heat transfer was enhanced by the roughness features and the enhancement increased with increasing Reynolds number.

Experimental investigation on friction factor in pipes with large roughness

This paper experimentally and theoretically investigated the differences on friction factor in rough pipes with large surface roughness. Deviation from theoretical theory for laminar flow in rough tubes was confirmed again. The product of fRe was larger than 64 and can be predicted as a quadratic equation of relative roughness. Earlier transition between flow regimes was observed. The critical Reynolds number between flow regimes decreased as the relative roughness increased. The range of Reynolds number for transitional flow regime gradually reduces with an increase in surface roughness. When relative roughness reach peak (Δ/d=1/2.4), transitional flow regime disappeared. Navier–Stokes equation was used to analyze the differences on flow behavior. It indicates flow obstruction of surface roughness caused curve flow for laminar flow in porous medium and linear law for laminar flow in porous media may be an approximate expression by neglecting the inertial forces at low Reynolds number.

Thermal and hydraulic characteristics of discretely rough tubes at transient flow regimes

Results are presented from an experimental study of heat transfer and hydraulic friction in tubes with discrete roughness in the form of annular protrusions and systems of spherical protrusions in forced convection of water. The thermohydraulic efficiency of these tubes is estimated.