alinik
۱۴ مهر ۱۳۸۸, ۰۰:۲۸
Abstract
In the present work, flow-induced vibration characteristics of conical-ring turbulators used
for heat transfer enhancement in heat exchangers are investigated experimentally. The conical-
rings, having 10, 20 and 30 mm pitches, are inserted in a model pipe-line through which
air is passed as the working fluid. Vortex-shedding frequencies and amplitude are determined
and St-Re, Prms-Re variations are presented graphically. Flow-acoustic coupling is also analyzed
experimentally. It is observed that as the pitch increases, vortex shedding frequencies
also increase and the maximum amplitudes of the vortices produced by conical-ring turbulators
occur with small pitches. In addition, the effects of the promoters on the heat transfer and
friction factor are investigated experimentally for all the arrangements. It is found that the
Nusselt number increases with the increasing Reynolds number and the maximum heat
transfer is obtained for the smallest pitch arrangement.
# 2003 Elsevier Ltd. All rights reserved.
Keywords: Turbulator; Heat transfer enhancement; Flow-induced vibration; Vortex shedding
1. Introduction
The development of high-performance thermal systems has increased interest in
heat transfer enhancement techniques. The use of artificially roughened surfaces,
extended surfaces, inlet vortex generators, vibration of the surface or fluid, application
of electrostatic fields, and the insertion in tubes of objects such as twisted
Applied Energy 78 (2004) 273–288
www.elsevier.com/locate/apenergy
0306-2619/$ - see front matter # 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.apenergy.2003.09.001
* Corresponding author. Tel.: +90-442-231-48-44; fax: +90-442-236-09-57.
E-mail addresses: kyakut@atauni.edu.tr (K. Yakut), bsahin@atauni.edu.tr (B. Sahin).
tapes, coiled wires, or spinners are a few examples of such enhancement techniques.
Existing systems can often be improved by using an enrichment method in some
applications such as the design of heat exchangers for use in space vehicles, while
an augmentative scheme may be mandatory in order for the system to function
properly and meet the size limitations imposedDevices establishing swirl or turbulence in the fluid are particularly attractive
augmentative schemes for forced convection systems. However, these devices placed
in the stream deteriorate the flow. Turbulators generate almost-periodic vortices in
the flow because of boundary layer separations. If the vortex shedding frequency is
coupled to the natural frequency of the system, continuing oscillations will cause
forced vibrations, i.e. ‘‘flow-induced vibrations’’. If the amplitude of the flowinduced
vibrations has a high value, some physical measurements such as flow rate,
velocity or pressure taken from the system may not be correct. Due to the almostperiodic
vortex structure that causes fatigue, some cracks may occur in the system
affected by the vibrations. In addition, energy loss increases lead to larger operating
costs.
To date, many studies have been conducted on passive heat-transfer enhancement
methods and fluid flow. Some researchers, [1–6] investigated the effect of swirling
flow produced by means of twisted tape. They reported that both the friction factor
and heat transfer coefficient increased substantially beyond a particular Reynolds
number. Hsieh et al. [7] undertook an experimental study for developing turbulent
mixed convection in a horizontal circular tube with strip-type inserts for different
Reynolds numbers. Also the heat transfer enhancement was found to be 2 to 3 times
bigger than for the bare tube. In the study performed by Yıldız et al. [8], twisted
narrow, thin metallic strips were placed in the inner pipe of a concentric double-pipe
heat exchanger. Tonkonogii et al. [9], studied fluid dynamics and heat transfer in
plane channels. They used different turbulators as a means of augmenting the fluidwall
heat-transfer.
When the literature is searched from point of view of flow-induced vibrations in
heat exchangers, it was noted that some stimulation mechanisms such as breaking
away of a vortex, pulsations, turbulence, fluid-elastic stimulation and acoustic resonance
in the flow around tube-bundles in heat exchangers have been investigated
[10–12]. These mechanisms sometimes caused system deformation by fatigue. While
fluid-elastic instability and periodic wake-shedding may cause failure in a very short
time, turbulence excitation may induce enough vibration response to cause longterm
fretting-wear damage [13]. Furthermore, the effect of tube-array geometries on
flow-induced vibrations was studied by Chen [14], Paidoussis [15], Granger and
Campistron [16], and Ziada and Oengoren [17]. In addition, many studies have been
made concerning the control valves, geometric transients and flow measurement
elements by considering the structural analysis of flow-induced vibrations generated
by discontinuity elements placed in pipelines [18,19]. Valve noises resulting from
flow-induced vibrations in control valves have been discussed previously [20–23].
While much progress has been accomplished in understanding flow-induced
vibration mechanisms, there are still some important areas requiring attention.
However, to the best of our knowledge, the effect of vortices from turbulators
inserted into tubes for heat transfer enhancement has not been examined in detail, so
far. In this study, analyses of heat transfer and friction loss have been investigated
by placing conical-ring turbulators into the tube that can be used to increase the
heat transfer in heat exchangers. Shedding frequencies and amplitudes of vortices
produced by conical-ring turbulators have also been determined by considering
K. Yakut, B. Sahin / Applied Energy 78 (2004) 273–288 275
system deformations that occur due to flow-induced vibrations in the pipelines,
which are widely used in engineering applications today. Finally, coupling of flow
(vortex)-acoustic structures has been investigated.
2. Experimental set-ups and procedure
2.1. Determination of heat transfer characteristics
The experimental set-up used to determine the heat transfer and friction-loss
characteristics is shown in Fig. 1. Initially, air entered the experimental set-up by a
centrifugal blower. The flow rate of the air was measured by an orifice meter produced
according to ASME standards [24]. The inner diameter of the test pipe of
1240 mm in length was 50 mm. All of the pipes and orifice meter were made of
aluminum. The test pipe was heated by winding continuously a flexible electricallyinsulated
heating wire around the pipe, which provided a constant heat flux boundary
condition. The heater output power was 58 W at 41 V and the measurement
current 1.41 A. The electrical power input to the heater was controlled by a variac
transformer to obtain a constant heat flux along the test tube. The outside of the test
pipe was insulated with a layer of glasswool. There was a mixing chamber with two
thermojunctions at the exit of the test section to measure the outlet bulk temperature
of the air.
The steady-state inlet and outlet bulk temperatures of the air, and the temperature
of the pipe wall at twenty stations were measured by 0.5 mm diameter copper-constantan
thermocouples. The readings of the thermocouples were recorded using a
computer via a data acquisition card (HG818 and 789D multiplexer), and the average
of these readings was taken to be the steady-state temperature of the test surface.
Fig. 1. Experimental heat transfer setup.
276 K. Yakut, B. Sahin / Applied Energy 78 (2004) 273–288
The inlet temperature of the air was measured at the inlet of the test section. The
outlet temperature of the air was taken as the average reading of the two copperconstantan
thermojunction located at the mixing chamber. The temperature of the
pipe surface was taken as the average of the temperatures from the 20 thermocouples.
The copper-constantan thermocouples were calibrated, to within0.1 C
deviation, in a thermostat before being used. In the experiments, 50–60 min elapsed
to reach a steady state.
The geometric structure of the conical-rings given in Fig. 2 was constructed from
St 42 steel to allow periodic redevelopment of the boundary layers. The total lengths
of the conical-rings were 1240 mm, while their big and small diameters were 44 and
24 mm, respectively. Three arrangements of the turbulators, having 10, 20 and 30
mm pitches, were prepared for the experiments.
The Reynolds number of the air ranging from 5000 to 20,000 was based on the
bulk mean properties and the pipe’s inner-diameter. All fluid properties were determined
at the overall bulk mean-temperature. Whenever the thermal equilibrium was
reached, the measured data were recorded. The pressure losses were measured with Utube
manometers with a special manometer fluid having a relative density of .
In the present work, flow-induced vibration characteristics of conical-ring turbulators used
for heat transfer enhancement in heat exchangers are investigated experimentally. The conical-
rings, having 10, 20 and 30 mm pitches, are inserted in a model pipe-line through which
air is passed as the working fluid. Vortex-shedding frequencies and amplitude are determined
and St-Re, Prms-Re variations are presented graphically. Flow-acoustic coupling is also analyzed
experimentally. It is observed that as the pitch increases, vortex shedding frequencies
also increase and the maximum amplitudes of the vortices produced by conical-ring turbulators
occur with small pitches. In addition, the effects of the promoters on the heat transfer and
friction factor are investigated experimentally for all the arrangements. It is found that the
Nusselt number increases with the increasing Reynolds number and the maximum heat
transfer is obtained for the smallest pitch arrangement.
# 2003 Elsevier Ltd. All rights reserved.
Keywords: Turbulator; Heat transfer enhancement; Flow-induced vibration; Vortex shedding
1. Introduction
The development of high-performance thermal systems has increased interest in
heat transfer enhancement techniques. The use of artificially roughened surfaces,
extended surfaces, inlet vortex generators, vibration of the surface or fluid, application
of electrostatic fields, and the insertion in tubes of objects such as twisted
Applied Energy 78 (2004) 273–288
www.elsevier.com/locate/apenergy
0306-2619/$ - see front matter # 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.apenergy.2003.09.001
* Corresponding author. Tel.: +90-442-231-48-44; fax: +90-442-236-09-57.
E-mail addresses: kyakut@atauni.edu.tr (K. Yakut), bsahin@atauni.edu.tr (B. Sahin).
tapes, coiled wires, or spinners are a few examples of such enhancement techniques.
Existing systems can often be improved by using an enrichment method in some
applications such as the design of heat exchangers for use in space vehicles, while
an augmentative scheme may be mandatory in order for the system to function
properly and meet the size limitations imposedDevices establishing swirl or turbulence in the fluid are particularly attractive
augmentative schemes for forced convection systems. However, these devices placed
in the stream deteriorate the flow. Turbulators generate almost-periodic vortices in
the flow because of boundary layer separations. If the vortex shedding frequency is
coupled to the natural frequency of the system, continuing oscillations will cause
forced vibrations, i.e. ‘‘flow-induced vibrations’’. If the amplitude of the flowinduced
vibrations has a high value, some physical measurements such as flow rate,
velocity or pressure taken from the system may not be correct. Due to the almostperiodic
vortex structure that causes fatigue, some cracks may occur in the system
affected by the vibrations. In addition, energy loss increases lead to larger operating
costs.
To date, many studies have been conducted on passive heat-transfer enhancement
methods and fluid flow. Some researchers, [1–6] investigated the effect of swirling
flow produced by means of twisted tape. They reported that both the friction factor
and heat transfer coefficient increased substantially beyond a particular Reynolds
number. Hsieh et al. [7] undertook an experimental study for developing turbulent
mixed convection in a horizontal circular tube with strip-type inserts for different
Reynolds numbers. Also the heat transfer enhancement was found to be 2 to 3 times
bigger than for the bare tube. In the study performed by Yıldız et al. [8], twisted
narrow, thin metallic strips were placed in the inner pipe of a concentric double-pipe
heat exchanger. Tonkonogii et al. [9], studied fluid dynamics and heat transfer in
plane channels. They used different turbulators as a means of augmenting the fluidwall
heat-transfer.
When the literature is searched from point of view of flow-induced vibrations in
heat exchangers, it was noted that some stimulation mechanisms such as breaking
away of a vortex, pulsations, turbulence, fluid-elastic stimulation and acoustic resonance
in the flow around tube-bundles in heat exchangers have been investigated
[10–12]. These mechanisms sometimes caused system deformation by fatigue. While
fluid-elastic instability and periodic wake-shedding may cause failure in a very short
time, turbulence excitation may induce enough vibration response to cause longterm
fretting-wear damage [13]. Furthermore, the effect of tube-array geometries on
flow-induced vibrations was studied by Chen [14], Paidoussis [15], Granger and
Campistron [16], and Ziada and Oengoren [17]. In addition, many studies have been
made concerning the control valves, geometric transients and flow measurement
elements by considering the structural analysis of flow-induced vibrations generated
by discontinuity elements placed in pipelines [18,19]. Valve noises resulting from
flow-induced vibrations in control valves have been discussed previously [20–23].
While much progress has been accomplished in understanding flow-induced
vibration mechanisms, there are still some important areas requiring attention.
However, to the best of our knowledge, the effect of vortices from turbulators
inserted into tubes for heat transfer enhancement has not been examined in detail, so
far. In this study, analyses of heat transfer and friction loss have been investigated
by placing conical-ring turbulators into the tube that can be used to increase the
heat transfer in heat exchangers. Shedding frequencies and amplitudes of vortices
produced by conical-ring turbulators have also been determined by considering
K. Yakut, B. Sahin / Applied Energy 78 (2004) 273–288 275
system deformations that occur due to flow-induced vibrations in the pipelines,
which are widely used in engineering applications today. Finally, coupling of flow
(vortex)-acoustic structures has been investigated.
2. Experimental set-ups and procedure
2.1. Determination of heat transfer characteristics
The experimental set-up used to determine the heat transfer and friction-loss
characteristics is shown in Fig. 1. Initially, air entered the experimental set-up by a
centrifugal blower. The flow rate of the air was measured by an orifice meter produced
according to ASME standards [24]. The inner diameter of the test pipe of
1240 mm in length was 50 mm. All of the pipes and orifice meter were made of
aluminum. The test pipe was heated by winding continuously a flexible electricallyinsulated
heating wire around the pipe, which provided a constant heat flux boundary
condition. The heater output power was 58 W at 41 V and the measurement
current 1.41 A. The electrical power input to the heater was controlled by a variac
transformer to obtain a constant heat flux along the test tube. The outside of the test
pipe was insulated with a layer of glasswool. There was a mixing chamber with two
thermojunctions at the exit of the test section to measure the outlet bulk temperature
of the air.
The steady-state inlet and outlet bulk temperatures of the air, and the temperature
of the pipe wall at twenty stations were measured by 0.5 mm diameter copper-constantan
thermocouples. The readings of the thermocouples were recorded using a
computer via a data acquisition card (HG818 and 789D multiplexer), and the average
of these readings was taken to be the steady-state temperature of the test surface.
Fig. 1. Experimental heat transfer setup.
276 K. Yakut, B. Sahin / Applied Energy 78 (2004) 273–288
The inlet temperature of the air was measured at the inlet of the test section. The
outlet temperature of the air was taken as the average reading of the two copperconstantan
thermojunction located at the mixing chamber. The temperature of the
pipe surface was taken as the average of the temperatures from the 20 thermocouples.
The copper-constantan thermocouples were calibrated, to within0.1 C
deviation, in a thermostat before being used. In the experiments, 50–60 min elapsed
to reach a steady state.
The geometric structure of the conical-rings given in Fig. 2 was constructed from
St 42 steel to allow periodic redevelopment of the boundary layers. The total lengths
of the conical-rings were 1240 mm, while their big and small diameters were 44 and
24 mm, respectively. Three arrangements of the turbulators, having 10, 20 and 30
mm pitches, were prepared for the experiments.
The Reynolds number of the air ranging from 5000 to 20,000 was based on the
bulk mean properties and the pipe’s inner-diameter. All fluid properties were determined
at the overall bulk mean-temperature. Whenever the thermal equilibrium was
reached, the measured data were recorded. The pressure losses were measured with Utube
manometers with a special manometer fluid having a relative density of .