**4.2 Approach II: auxiliary heat exchanger**

In the second methodology for variable wall heat flux conditions, a tube-in-tube or shell-and-tube heat exchanger installed at the test section may be used for measuring and controlling local vapor quality at the outlet of test tube. Analogous to the earlier cases, the inlet vapor quality is measured and controlled by monitoring the calibrated heat supplied by the Pre-Heater using the Eq. (2).

As represented in **Figure 4**, a hot liquid single-phase flow with known mass flow rates and known temperatures and pressures at the inlet and outlet passes through

**Figure 3.**

*Heat Transfer - Design, Experimentation and Applications*

the cold-side fluid (i.e. internal flow boiling).

**4.1 Approach I: auxiliary after-heater**

of surface heat flux provided by heating tapes wrapped around the test section. However, the use of hot fluid heating rather than electrical heating to generate constant wall temperature conditions does not allow the direct control of outlet vapor quality due to the unknown variable surface heat flux exchanged between the hot-side fluid (e.g. external condensation of steam or single-phase hot liquid) and

**Figure 3** illustrates a typical case of constant temperature boundary conditions imposed by external condensation of steam on the test tube. The test section shown in this case is the place where external condensation and internal flow boiling occur simultaneously. The test section therefore functions as a cross flow heat exchanger whose both sides are manipulated with phase-change heat transfer processes. The test apparatus for this arrangement is to consist of two closed loops, including: external condensation loop (i.e. steam condensation over a horizontal tube) and

Within the external condensation loop, saturated vapor of water at saturation temperature and pressure of Tsat,steam and Psat,steam is provided by a steam generator and then enters the test chamber. After condensation of steam on the horizontal test tube due to the temperature difference between the saturated vapor and the tube surface (called subcooling), the condensate is driven by gravity and collected in a condensate reservoir to feed the steam generator and set a steady flow circulation in the external condensation loop. Regarding the internal boiling loop, the fluid is warmed up by the SP-Heater in order to reach the saturated liquid state (*x* = 0) at the saturation temperature and pressure of Tsat and Psat, respectively (Tsat < Tsat,steam). Using the Pre-Heater located right before the test section, the saturated liquid therefore reaches a certain vapor quality at the inlet of the test section (*x*in) and is afterwards exposed to the latent heat released from the external condensation side

The unknown outlet vapor quality can be measured by adding a calibrated heat-supplying unit (After-Heater) with power controller installed right after the test section in order to take the two-phase flow with unknown outlet quality to the known state of saturated vapor (i.e. *x* = 100%) at the same saturation temperature

*Q Q Q mh h calib after suppl after loss after* − −− = −=− ( ) ( *g x*( <sup>=</sup>1) *x out* [ ] ) (4)

where *Qsuppl after* <sup>−</sup> stands for the heat experimentally supplied by the After-Heater, *Qloss after* <sup>−</sup> is the corresponding heat loss from this heat-supplying unit, and *Qcalib after* <sup>−</sup> accounts for the calibrated heat which is actually transferred to the boiling flow. Having the enthalpy of saturated vapor (*hg* (x=1)) known, the only unknown parameter in Eq. (4) is the enthalpy at the outlet of the test section ( *x out* [ ] *h* ) from which the outlet vapor quality can be extracted at the operating saturation temperature and pressure. Similar to the case of constant wall heat flux boundary conditions stated earlier, the inlet vapor quality can independently be measured and controlled by adjusting the calibrated heat supplied by the Pre-Heater using the Eq. (2).

In this approach, to ensure the state of saturated vapor, the After-Heater located after the test section is adjusted to supply the required latent heat for the two-phase flow with a certain outlet quality to reach a temperature slightly higher than the constant saturation temperature, which would be the starting point of the superheated vapor state. As shown in **Figure 3**, using the sight glass installed after the

internal boiling loop (i.e. two-phase flow boiling inside the tube).

to reach an unknown higher vapor quality at the outlet (*x*out).

of Tsat. In this case, the energy balance is dictated as follows:

**394**

*The experimental approach to measuring vapor qualities for uniform wall temperature boundary conditions.*

**Figure 4.**

*The experimental approach to measuring vapor qualities for variable wall heat flux boundary conditions.*

the outer tube (shell side), while the boiling flow of a known inlet vapor quality with a saturation temperature (Tsat) lower than that of the heating liquid (Th) enters the inner tube of the counter-flow heat exchanger. After latent heat acquisition from the hot-side fluid, the internal boiling flow undergoes an unknown increase in vapor quality at the outlet of test tube whereas the heating liquid in the shell side experiences a temperature reduction as a result of sensible heat rejection yet its temperature at the outlet remains higher than the constant saturation temperature of internal boiling flow. The vapor quality at the outlet of the test tube can therefore be controlled by adjusting the mass flow rate of the hot liquid single-phase flow(*mh* ) at the shell side of the counter-flow heat exchanger.

The amounts of heat exchanged between the internal boiling flow and the heating liquid can be measured by writing down an energy balance as follows:

$$\dot{Q}\_{\text{sejectted}} = \dot{m}\_h \, \mathbf{C}\_p \left( T\_{h,in} - T\_{h,out} \right) = Q\_{\text{gained}} = \dot{m} \left( h\_{\text{x}[out]} - h\_{\text{x}[in]} \right) \tag{5}$$

Aside from *x out* [ ] *h* , all the other parameters in Eq. (5) are known. The enthalpy at the outlet of the test section ( *x out* [ ] *h* ) can thus be calculated and the vapor quality at the outlet of the test tube can be measured and controlled subsequently. Similar to the approach engaged to the uniform wall heat flux boundary conditions, the outlet vapor quality derived from this approach contains an accumulated error arisen from earlier measurement of the inlet vapor quality.

To keep the boiling fluid recirculated, this is evident that other components are required for the internal boiling loop, which are not shown in **Figures 2–4**. Subsequent to the test section or the After-Heater, the two-phase flow with a certain outlet quality or the saturated vapor is required to be condensed in a heat exchanger to reach the state of saturated liquid which is followed by a drop in temperature and pressure after passing through an expansion valve to reach the state of subcooled liquid prior to entering the pump in order to avoid the cavitation phenomenon. The liquid flow is then squeezed by a gear pump up to the desired saturation pressure to enter the SP-Heater.

#### **5. In-situ measurement for any thermal boundary conditions**

Regardless of the type of thermal boundary conditions governed on the test section, the local vapor quality of a two-phase flow boiling may be obtained through in-situ measurements.

Using the experimental approaches and/or instruments introduced here, first, the local density of two-phase flow at either of the inlet or outlet of a test section can be measured in-situ for any thermal boundary conditions that might be imposed on the test section. After obtaining the density, two independent thermodynamic properties of the flow at either inlet or outlet are known (*i.e.* density and either of saturation temperature or corresponding pressure) in order to look up the enthalpy of the two-phase flow at either inlet or outlet ( *x in*[ ] *h* or *x out* [ ] *h* ). Having the local enthalpies known, the local vapor quality (*x*) can be readily obtained via *h h xh <sup>x</sup>* = + *f x*( <sup>=</sup>0) *fg* as the only unknown parameter left here. However, this is important to note that the accuracy of this approach is lower than those of the earlier approaches described so far in the present study due to the less accuracy of the limited experimental methodologies [16] and instruments [17–19] introduced to date to measure density of a two-phase flow.

Interest in the determination of two-phase flow density has brought about the design and development of various instruments to measure density in cryogenic flow

**397**

**Figure 5.**

*Experimental Approaches to Measurement of Vapor Quality of Two-Phase Flow Boiling*

systems. The more promising of the methods suggested are based on either (i) measurements of the average dielectric constant or capacitance of the two-phase fluid or (ii) measurements of the nuclear radiation attenuation properties of the two-phase fluid. In principle, both of these measurable quantities are associated with the fluid density.

of liquid and two-phase hydrogen flow. Most of their measured and calculated values of density exhibited a deviation up to ±15% of the full-scale density. The advanced Coriolis meters have also been investigated for measurement of twophase flow density [18, 19]. In this context, Reizner [18] has addressed the issues concerned to metering two-phase flow using the Coriolis meters. Technically, this is hard to retain flow-tube oscillations within two-phase flow due to the high and rapid damping of oscillations which is, by far, up to three orders of magnitude higher than that of the single-phase flow. Once the transmitter is not capable of maintaining the oscillations, the Coriolis meter is found to be "stalled", and no measurements are provided. Even in the case of averting the stalling, large errors in

Although there is no specific instrument to accurately measure density of a two-phase flow, the technique(s) recently introduced by Boltenko [16] can measure the density with a reasonable accuracy. The range of uncertainty reported for his

The following is a brief explanation of the proposed techniques to measure local

i.*Gamma-raying Technique:* The method of Gamma-raying makes it possible to measure density of a two-phase flow both in steady and transient flow regimes as well as makes it possible to carry out ongoing record of *ρ(t)* with averaging over the time intervals which are remarkably shorter than the typi-

Turney and Snyder [17] used a capacitance density meter to measure the density

*DOI: http://dx.doi.org/10.5772/intechopen.94473*

measurements of mass flow and density are induced.

cal duration of an unsteady process (τ > 0.1 s) [20].

*Schematic of the hydrostatic method to determine density of a two-phase flow [16].*

technique(s) is between 3% and 5%.

density of a two-phase flow:

#### *Experimental Approaches to Measurement of Vapor Quality of Two-Phase Flow Boiling DOI: http://dx.doi.org/10.5772/intechopen.94473*

systems. The more promising of the methods suggested are based on either (i) measurements of the average dielectric constant or capacitance of the two-phase fluid or (ii) measurements of the nuclear radiation attenuation properties of the two-phase fluid. In principle, both of these measurable quantities are associated with the fluid density.

Turney and Snyder [17] used a capacitance density meter to measure the density of liquid and two-phase hydrogen flow. Most of their measured and calculated values of density exhibited a deviation up to ±15% of the full-scale density. The advanced Coriolis meters have also been investigated for measurement of twophase flow density [18, 19]. In this context, Reizner [18] has addressed the issues concerned to metering two-phase flow using the Coriolis meters. Technically, this is hard to retain flow-tube oscillations within two-phase flow due to the high and rapid damping of oscillations which is, by far, up to three orders of magnitude higher than that of the single-phase flow. Once the transmitter is not capable of maintaining the oscillations, the Coriolis meter is found to be "stalled", and no measurements are provided. Even in the case of averting the stalling, large errors in measurements of mass flow and density are induced.

Although there is no specific instrument to accurately measure density of a two-phase flow, the technique(s) recently introduced by Boltenko [16] can measure the density with a reasonable accuracy. The range of uncertainty reported for his technique(s) is between 3% and 5%.

The following is a brief explanation of the proposed techniques to measure local density of a two-phase flow:

i.*Gamma-raying Technique:* The method of Gamma-raying makes it possible to measure density of a two-phase flow both in steady and transient flow regimes as well as makes it possible to carry out ongoing record of *ρ(t)* with averaging over the time intervals which are remarkably shorter than the typical duration of an unsteady process (τ > 0.1 s) [20].

**Figure 5.** *Schematic of the hydrostatic method to determine density of a two-phase flow [16].*

*Heat Transfer - Design, Experimentation and Applications*

flow(*mh* ) 

enter the SP-Heater.

in-situ measurements.

date to measure density of a two-phase flow.

the outer tube (shell side), while the boiling flow of a known inlet vapor quality with a saturation temperature (Tsat) lower than that of the heating liquid (Th) enters the inner tube of the counter-flow heat exchanger. After latent heat acquisition from the hot-side fluid, the internal boiling flow undergoes an unknown increase in vapor quality at the outlet of test tube whereas the heating liquid in the shell side experiences a temperature reduction as a result of sensible heat rejection yet its temperature at the outlet remains higher than the constant saturation temperature of internal boiling flow. The vapor quality at the outlet of the test tube can therefore

be controlled by adjusting the mass flow rate of the hot liquid single-phase

The amounts of heat exchanged between the internal boiling flow and the heat-

Aside from *x out* [ ] *h* , all the other parameters in Eq. (5) are known. The enthalpy at the outlet of the test section ( *x out* [ ] *h* ) can thus be calculated and the vapor quality at the outlet of the test tube can be measured and controlled subsequently. Similar to the approach engaged to the uniform wall heat flux boundary conditions, the outlet vapor quality derived from this approach contains an accumulated error

To keep the boiling fluid recirculated, this is evident that other components are required for the internal boiling loop, which are not shown in **Figures 2–4**. Subsequent to the test section or the After-Heater, the two-phase flow with a certain outlet quality or the saturated vapor is required to be condensed in a heat exchanger to reach the state of saturated liquid which is followed by a drop in temperature and pressure after passing through an expansion valve to reach the state of subcooled liquid prior to entering the pump in order to avoid the cavitation phenomenon. The liquid flow is then squeezed by a gear pump up to the desired saturation pressure to

( ) ( [ ] [ ] ) . , , *Q C T T Q mh h rejected mh p h in h out gained x out x in* = −= = − (5)

at the shell side of the counter-flow heat exchanger.

ing liquid can be measured by writing down an energy balance as follows:

arisen from earlier measurement of the inlet vapor quality.

**5. In-situ measurement for any thermal boundary conditions**

Regardless of the type of thermal boundary conditions governed on the test section, the local vapor quality of a two-phase flow boiling may be obtained through

Using the experimental approaches and/or instruments introduced here, first, the local density of two-phase flow at either of the inlet or outlet of a test section can be measured in-situ for any thermal boundary conditions that might be imposed on the test section. After obtaining the density, two independent thermodynamic properties of the flow at either inlet or outlet are known (*i.e.* density and either of saturation temperature or corresponding pressure) in order to look up the enthalpy of the two-phase flow at either inlet or outlet ( *x in*[ ] *h* or *x out* [ ] *h* ). Having the local enthalpies known, the local vapor quality (*x*) can be readily obtained via *h h xh <sup>x</sup>* = + *f x*( <sup>=</sup>0) *fg* as the only unknown parameter left here. However, this is important to note that the accuracy of this approach is lower than those of the earlier approaches described so far in the present study due to the less accuracy of the limited experimental methodologies [16] and instruments [17–19] introduced to

Interest in the determination of two-phase flow density has brought about the design and development of various instruments to measure density in cryogenic flow

**396**

ii.*Hydrostatic Technique:* The hydrostatic method to determine the density of a two-phase flow is performed by the measurement of static pressures at two points of a channel. After measuring the static pressure difference between these two pressure tapping points, the average density of the two-phase flow can be obtained by the following correlation [21]:

$$\overline{\rho} = \frac{\left(\Delta P\_{\text{st}} - \Delta P\_{\text{hyd}}\right)}{\text{g } H} \tag{6}$$

in which *g* is the gravitational acceleration, *H* is the pipe diameter, *ΔP*hyd accounts for the hydraulic resistance between the pressure tap points, and *ΔP*st stands for the hydrostatic pressure difference between the pressure tap points.

As can be seen from Eq. (6), it is possible to obtain ρ only if *ΔP*hyd is known. Hence, the hydrostatic technique may be employed to measure ρ for test sections with horizontal orientation. In the case of horizontal test tube *ΔP*hyd = 0, and then Eq. (6) reduces to:

$$
\overline{\rho} = \frac{\Delta P\_s}{\text{g } H} \tag{7}
$$

**399**

*Experimental Approaches to Measurement of Vapor Quality of Two-Phase Flow Boiling*

**6.2 Remarks on the approach I for constant wall temperature conditions**

Using the approach I described in Section 4.1, higher accuracy and lower uncertainty in vapor quality measurements can be achieved by conducting accurate estimation of heat losses as well as accurate calibration of heat supplies. Furthermore, measurement of local vapor quality at the outlet of test section using this technique does not contain any accumulated errors arising from earlier measurements of inlet vapor quality as represented in Eq. (4). This is while the approach is very affordable

Taking advantage of the After-Heater located after the test section, this technique does not interfere with heat transfer data collected from the test section and does not pose the issues of circumferential and axial heat conduction caused by electrical heating for stratified and annular flow patterns within the

The approach II described in Section 4.2 is more expensive than the earlier techniques presented. The method is also not as simple as the earlier techniques in implementation. Using this approach, there is still accumulated error in measurement of outlet vapor quality arisen from the earlier measurement of inlet vapor

Moreover, the main drawback is that the methodology is likely to pose a higher overall uncertainty in measuring the local vapor qualities as compared to the earlier techniques described in Sections 3 and 4.1 since there will be higher number of points to be measured for temperature, pressure, and mass flow rate as indicated in Eq. (5). In this technique, five more precision instruments are required to be in service in order to measure flow rate of the hot-side fluid (one flow sensor), pressures (two pressure transducers), and temperatures (two thermocouple probes) at

**6.3 Remarks on the approach II for variable wall heat flux conditions**

**6.4 Remarks on the approaches for any thermal boundary conditions**

The major drawback of the in-situ measurements is that the techniques and/ or instruments introduced to date pose a low accuracy to measure local density of a two-phase flow, which ultimately makes the overall uncertainty for vapor quality measurements undesirable. In addition, very accurate and expensive pressure transducers and/or expensive advanced Coriolis meters are required to be procured

Vapor quality plays a key role in flow boiling heat transfer behavior and can noticeably affect the local flow boiling heat transfer coefficient. To accurately investigate the effect of vapor quality on flow boiling behavior, accurate measurement of

and control vapor quality for flow boiling tests and were classified based on the type of thermal boundary conditions induced on the test tube wall. Moreover, insitu measurements and techniques were also investigated to measure local density of two-phase flow and subsequently local vapor quality regardless of the governing

In the present study, various experimental techniques were presented to measure

the inlet and outlet of the shell side of heat exchanger.

to implement this technique properly.

local vapor quality is critical.

thermal boundary conditions.

**7. Conclusions**

*DOI: http://dx.doi.org/10.5772/intechopen.94473*

and simple in execution.

quality, according to Eq. (5).

test section.

**Figure 5** depicts schematics of the hydrostatic technique to measure the average density of a two-phase flow in a horizontal pipe.

#### **6. Merits and demerits**

In this section, the experimental approaches are sought to be compared based on their level of accuracy in measurement, affordability, and simplicity in implementation. Remarks, including merits and demerits, are expressed for each experimental technique described in the earlier sections in sequence.

#### **6.1 Remarks on the approach for uniform wall heat flux conditions**

The approach described in Section 3 is restricted to the investigation of the impact of local vapor quality on the heat transfer performance under known constant wall heat flux boundary conditions. Although the method is very affordable and simple to be implemented, accuracy of this methodology to measure local vapor quality is reliant heavily on the accuracy in estimating heat losses and calibrating heat supplies. Furthermore, it is important to note that the measurement of local vapor quality at the outlet of test section using this technique contains an accumulated error arisen from earlier measurement of local vapor quality at the inlet according to Eq. (3).

On the other hand, the measurement of flow boiling heat transfer data for horizontal test tubes using electrical heating has always been a subject of debate [22], where hot fluid heating is preferred to be used. In this regard, the following concerns are needed to be addressed: (i) for different types of stratified flow pattern, hot fluid heating induces practically uniform wall temperature boundary conditions for the tube perimeter, whereas electrical heating contributes to the circumferential heat conduction for the tube perimeter from the hot, dry-wall conditions at the top to the colder, wet-wall conditions at the bottom of the tube, leading to unknown thermal boundary conditions, (ii) for annular flow pattern with partial dryout at the top of the tube, electrical heating is not also advised due to the axial heat conduction along the tube.

*Experimental Approaches to Measurement of Vapor Quality of Two-Phase Flow Boiling DOI: http://dx.doi.org/10.5772/intechopen.94473*

#### **6.2 Remarks on the approach I for constant wall temperature conditions**

Using the approach I described in Section 4.1, higher accuracy and lower uncertainty in vapor quality measurements can be achieved by conducting accurate estimation of heat losses as well as accurate calibration of heat supplies. Furthermore, measurement of local vapor quality at the outlet of test section using this technique does not contain any accumulated errors arising from earlier measurements of inlet vapor quality as represented in Eq. (4). This is while the approach is very affordable and simple in execution.

Taking advantage of the After-Heater located after the test section, this technique does not interfere with heat transfer data collected from the test section and does not pose the issues of circumferential and axial heat conduction caused by electrical heating for stratified and annular flow patterns within the test section.

#### **6.3 Remarks on the approach II for variable wall heat flux conditions**

The approach II described in Section 4.2 is more expensive than the earlier techniques presented. The method is also not as simple as the earlier techniques in implementation. Using this approach, there is still accumulated error in measurement of outlet vapor quality arisen from the earlier measurement of inlet vapor quality, according to Eq. (5).

Moreover, the main drawback is that the methodology is likely to pose a higher overall uncertainty in measuring the local vapor qualities as compared to the earlier techniques described in Sections 3 and 4.1 since there will be higher number of points to be measured for temperature, pressure, and mass flow rate as indicated in Eq. (5). In this technique, five more precision instruments are required to be in service in order to measure flow rate of the hot-side fluid (one flow sensor), pressures (two pressure transducers), and temperatures (two thermocouple probes) at the inlet and outlet of the shell side of heat exchanger.

#### **6.4 Remarks on the approaches for any thermal boundary conditions**

The major drawback of the in-situ measurements is that the techniques and/ or instruments introduced to date pose a low accuracy to measure local density of a two-phase flow, which ultimately makes the overall uncertainty for vapor quality measurements undesirable. In addition, very accurate and expensive pressure transducers and/or expensive advanced Coriolis meters are required to be procured to implement this technique properly.

### **7. Conclusions**

*Heat Transfer - Design, Experimentation and Applications*

can be obtained by the following correlation [21]:

As can be seen from Eq. (6), it is possible to obtain

density of a two-phase flow in a horizontal pipe.

technique described in the earlier sections in sequence.

**6.1 Remarks on the approach for uniform wall heat flux conditions**

Eq. (6) reduces to:

**6. Merits and demerits**

inlet according to Eq. (3).

conduction along the tube.

Hence, the hydrostatic technique may be employed to measure

ρ

in which *g* is the gravitational acceleration, *H* is the pipe diameter, *ΔP*hyd accounts for the hydraulic resistance between the pressure tap points, and *ΔP*st stands for the hydrostatic pressure difference between the pressure tap points.

with horizontal orientation. In the case of horizontal test tube *ΔP*hyd = 0, and then

ρ

*Pst g H*

**Figure 5** depicts schematics of the hydrostatic technique to measure the average

In this section, the experimental approaches are sought to be compared based on their level of accuracy in measurement, affordability, and simplicity in implementation. Remarks, including merits and demerits, are expressed for each experimental

The approach described in Section 3 is restricted to the investigation of the impact of local vapor quality on the heat transfer performance under known constant wall heat flux boundary conditions. Although the method is very affordable and simple to be implemented, accuracy of this methodology to measure local vapor quality is reliant heavily on the accuracy in estimating heat losses and calibrating heat supplies. Furthermore, it is important to note that the measurement of local vapor quality at the outlet of test section using this technique contains an accumulated error arisen from earlier measurement of local vapor quality at the

On the other hand, the measurement of flow boiling heat transfer data for horizontal test tubes using electrical heating has always been a subject of debate [22], where hot fluid heating is preferred to be used. In this regard, the following concerns are needed to be addressed: (i) for different types of stratified flow pattern, hot fluid heating induces practically uniform wall temperature boundary conditions for the tube perimeter, whereas electrical heating contributes to the circumferential heat conduction for the tube perimeter from the hot, dry-wall conditions at the top to the colder, wet-wall conditions at the bottom of the tube, leading to unknown thermal boundary conditions, (ii) for annular flow pattern with partial dryout at the top of the tube, electrical heating is not also advised due to the axial heat

ii.*Hydrostatic Technique:* The hydrostatic method to determine the density of a two-phase flow is performed by the measurement of static pressures at two points of a channel. After measuring the static pressure difference between these two pressure tapping points, the average density of the two-phase flow

> ( *P P st hyd* ) *g H*

∆ −∆ <sup>=</sup> (6)

only if *ΔP*hyd is known.

for test sections

ρ

<sup>∆</sup> <sup>=</sup> (7)

ρ

**398**

Vapor quality plays a key role in flow boiling heat transfer behavior and can noticeably affect the local flow boiling heat transfer coefficient. To accurately investigate the effect of vapor quality on flow boiling behavior, accurate measurement of local vapor quality is critical.

In the present study, various experimental techniques were presented to measure and control vapor quality for flow boiling tests and were classified based on the type of thermal boundary conditions induced on the test tube wall. Moreover, insitu measurements and techniques were also investigated to measure local density of two-phase flow and subsequently local vapor quality regardless of the governing thermal boundary conditions.

To provide a deeper insight to select an appropriate technique depending on researchers' choices, the experimental techniques were also compared based on their level of accuracy in measurement, affordability, and simplicity in implementation through addressing their potential weaknesses and strengths.
