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Frost accumulation control on an upward-facing horizontal flat plate using electric field.

INTRODUCTION

Frost formation and accumulation on the external surface of an evaporator is a very common issue because the thermodynamic conditions at which the heat exchanger works are often characterized by temperature (refrigerant side), temperature and humidity (moist air side) such that frost formation and accumulation can occur (namely, evaporation temperature lower than 0[degrees]C and evaporation temperature lower than air dew temperature). Frost accumulation involves an increase of the thermal insulation and of the air side pressure drop, whose consequences are the reduction of the heat exchanged between the air and the evaporator due to both the thermal resistance increase and the air velocity reduction. As a consequence, the evaporation pressure reduces causing a decrease of the C.O.P., a greater frost accumulation and a further pressure reduction, eventually up to requiring defrosting. To avoid shutdown danger, the frost is periodically removed from evaporative coil by defrosting cycles that, however, involves the service interruption and conspicuous energy consumption. Consequently, a technology able to reduce or control frost formation would be very effective in improving system performances, both from the point of view the air-side pressure drop reduction and the defrosting cycle number reduction. As explained in the next section, the EHD (ElectroHydroDynamic) technique, the application of an electric field to a frosting surface, could be that controlling technology.

FROST FORMATION UNDER THE ACTION OF AN ELECTRIC FIELD

Frost formation on a surface without a superimposed electric field is a quite well known phenomenon (Hayashi 1977) mainly influenced by moist air humidity ratio and the temperature difference between the air stream and the cold surface. This phenomenon changes considerably under the action of an electric field as shown by Ma and Peterson (1995) who carried out theoretical analyses oriented to understand the influence of an electric field on frost formation. Using the hypotheses that describe the behavior of the moist air and the theory of the phase change, the authors deduce the two following expressions which relate the saturation pressure of the water vapor and the frost crystal critical radius of nucleation with the electric field intensity:

[p/[p.sub.0]] = exp[[[lambda].sub.VS]/R(1/[T.sub.0] - 1/T) + [1/[RT]]([d[G.sub.0]]/[[dm.sub.F]] - [[[epsilon].sub.0]([[epsilon].sub.R] - 1)[E.sup.2]]/[[2[rho].sub.I]])] (1)

[r.sub.C] = [[2[gamma]]/[[[rho].sub.I][[RT]ln(p/[p.sub.0]) + [[[lambda].sub.VS]/R](1 - T/[T.sub.0]) + [[[epsilon].sub.0]([[epsilon].sub.R] - 1)[E.sup.2]]/[2[[rho].sub.I]]]]] (2)

The two equations above show that the action of the electric field causes, at the same temperature, the reduction of water vapor saturation pressure (1) and the decrease of the critical radius of nucleation of frost crystals (2). Both these effects promote frost formation because they increase the saturation pressure difference between the water vapor in the free stream and the saturated water vapor near frosted surface and they promote the formation of frost solid crystals.

Moreover, Babakin (1985) experimentally correlated the electric field intensity in proximity of the tip of a frost crystal with the crystal height to diameter ratio. The results show that the increase of the second parameter (crystal height to diameter ratio) causes an increase of the first (electric field intensity) and this, according to equation (1), causes a further reduction of the water vapor saturation pressure in proximity of the crystal tip and a local increase of the frost formation that further promotes the increase of the height to diameter ratio of the crystal.

Finally, beside this, it has to be considered that the electric field gradient is more intense in the direction of the electric field and therefore, near to the frost crystal tip, it creates a preferential direction, the direction of the electric field itself, along which the water vapor saturation pressure diminishes more quickly. Consequently, the frost crystals mostly grow along this direction, generating structures characterized by a high height to diameter ratio (that promotes the frost formation) and by a nearly complete absence of lateral branches.

From the discussion above it is possible to conclude that under the action of an electric field frost formation is promoted (reduction of both saturation pressure and critical radius of nucleation) and frost crystal morphology changes (height to diameter ratio). Figure 1 (from Libbrecht 1999) illustrates the frost crystal morphology in absence and presence of the electric field showing that the crystals growing under the action of the electric field are needled-shaped, longer and thinner crystals when compared to the analogous ones solidified without a superimposed electric field. As a consequence, mechanical structure of crystals is fragile and frost crystals can break up under the action of self-weight, viscous stress and electric stress, where the latter is expressed as follow:

[f.sub.E] = [[rho].sub.C]E - [1/2][E.sup.2][nabla][epsilon] + [1/2][nabla][[E.sup.2][rho][([[partial derivative][epsilon]]/[[partial derivative][rho]]).sub.T]] (3)

[FIGURE 1 OMITTED]

It is possible to observe that, as a function of the electric field intensity, there is a conflicting trend due to the fact that with the increase of the electric field intensity, both enhancement mechanism (equation 1 and 2) and electric stress, one of the reduction mechanisms (equation 3), increase leading to a trade-off situation. Therefore, this simple theoretical analysis states, and the experimental results confirm (see further in the text), that there exist a value of the electric field intensity that maximizes the effect of the frost reduction and after which the increase of the electric field intensity causes an increase of the heat and mass transfer mechanisms which reduce up to annul the frost reduction mechanism.

LITERATURE REVIEW

Experimental results available in the open literature about frost formation under the action of an electric field are quite limited and, with only one exception, they relate to simple geometry (flat surface or cylindrical pipe).

Schaefer (1950) obtained the first qualitative observations on the influence of an electric field on the frost formation and found that the presence of an electric field of high intensity increases the velocity of frost crystals growth while the structure in which they solidify is needled-shaped.

Subsequently, Marshall and Gunn (1957) observed the chaotic frost crystal growth characterized by numerous lateral ramifications under the action of an electric fields of modest intensity.

Bartlett et al. (1963) studied the frost crystal growth on a cylindrical surface within a radial electric field. They found the existence of a threshold value for the intensity of the field applied (equal to 50 kV/m or 15.3 kV/ft) beyond which the crystal solidification velocity comes approximately to 80 [micro]m/s (0.003 in/s) (between the ten and one hundred times greater than the normal growth velocity) and the shape that crystals assume is sharp, thin and branchless. The authors observed moreover the spontaneous break of the crystals and the migration of fragments towards the electrode opposite to the cold surface as an indication of the presence of an electric charge on the crystals. Such phenomenon appeared emphasized by the sudden reversal of the electric field polarity while it was independent from the polarity itself or the frequency of the field.

Maybank et al. (1967) analyzed the influence of an electric field on the frost crystal growth. They found that, for electric field intensity greater than 20 kV/m (6.1 kV/ft), the crystal growth velocity increases while the crystals are more numerous and they solidify in thinner and more fragile structures compared to the crystals formed without a superimposed electric field.

Chuang et al. (1971) investigated the frost formation on a vertical flat surface under the natural convection regime and exposed to an electric field created from a bare wire disposed with its axis parallel to the cold surface. They found an increase of 200% of the frost mass in correspondence of a corona current equal to 200 [micro]A.

Meng et al. (1990) studied the influence of an electric field on the frost formation on a horizontal flat plate (both single wire and set of many wire parallel). The results were not in agreement between themselves because, when the electric field is created by a single wire, an increase of 580% of the heat transfer coefficient and 400% of the mass transfer coefficient is found, while when the electric field is created by the wire set an increase of 250% of the heat transfer coefficient and a reduction of 40% of the mass transfer coefficient is measured.

Munakata et al. (1993) analyzed the frost formation over an horizontal flat plate under the natural convection regime in presence of an electric field sustained by a net electrode. The authors found out that, depending on the applied voltage, the frost mass accumulated on the cold surface could be greater or smaller than the mass accumulated in absence of the electric field. This indicates the existence of an electrical potential that could maximize the frost mass reduction and the value of reduction is 24% is obtained in the conditions of air temperature 17[degrees]C (63[degrees]F), humidity ratio 0.0085 kg/[kg.sub.DA], electrical potential 7.5 kV and test duration equal to 2 hours.

Blanford et al. (1995) investigated the frost formation on the surface of a finned heat exchanger under the action of an electric field created by bare wires placed within the exchanger. The authors reported a reduction of the frost mass equal to 20% when the corona currents intensity is lower than 5 [micro]A and an increase of the frost mass of the order of 100% when the corona currents intensity is approximately equal to 120 [micro]A.

Bloshteyn et al. (1999) studied the influence of an electric field on the frost formation on a vertical flat plate under the natural convection regime. They measured a reduction of mass approximately equal to 30%.

Molki et al. (2000) applied an intermittent electric field to a vertical surface under the natural convection regime. They found that the intermittent electric field causes an avalanche destruction of frost crystals and that an increase of the frequency of the field increases the effectiveness in removing frost.

Ohadi et al. (2002) carried out an extended experimental campaign in order to estimate the influence of both DC and AC electric fields on the frost formation over a flat plate arranged both vertically and horizontally. The flow regime was natural convection in the first case and both natural and forced convection in the second. The authors observed that when the surface is vertically oriented, the DC electric field reduces frost mass up to value equal to 30.8% [+ or -] 5% (applied voltage 35 kV) or equal to 34.7% [+ or -] 6.5% when the electric field is cyclically applied (10 minutes field is applied and 1 minute field is not applied). When the surface is horizontally oriented, the frost mass is reduced up to value equal to 4.5%/28% depending on the electrode used (parallel wire, zig-zag wire or net) and on the polarity (positive or negative). The influence of an AC electric field is analyzed only in the case of natural convection and vertically arranged surface. The obtained results show that the electric field reduces the frost thickness up to value of 22%/65% (frequency equal to 1000 Hz) while the frost mass accumulated on the surface increases up to value of 14.5%/44% if the electric field is applied with continuity for 2 hours, and diminishes up to value of 13.5%/35.2% if the electric field is applied for a minute after 30, 60 or 120 minutes.

Tudor et al. (2003) investigated the influence of the electric field intensity on the frost formation on the downward-facing surface of a horizontal flat plate in forced convection regime for DC electric fields and in natural and forced convection regime for AC electric fields. In both cases, the electric field is created by insulated wires parallel to the surface. When DC electric field is applied, the obtained results show a mass reduction equal to 10% (applied voltage equal to 7.5 kV), while when AC electric field is applied, the authors obtained a mass reduction equal to 36% (applied voltage equal to 14.5 kV) when the field is applied at the end of the experiment or at regular intervals and a mass increase equal to 44% (same applied voltage) when the field is applied for all the test duration. The forced convection results show the same results.

Wang et al. (2004) studied the influence of the polarity of a DC electric field on the frost formation on vertical flat fins in natural convection regime. The observations show that in the presence of an electric field the void degree of the surface increases with a consequent increase of the surface temperature of the heat exchanger (increase equal to 1[degrees]C/2[degrees]C or 1.8[degrees]F/3.6[degrees]F). Moreover, a reduction of frost thickness of 14% was observed for electric potential equal to 15 kV whereas an increment of the thickness of approximately 20% was observed when electric potential was set to - 15 kV. Although the authors did not estimate the frost mass deposited on the fin surface, they found that the frequency of the frost crystal breaking up was greater with negative polarity of electric field, 30%/50% greater than the frequency under positive polarity at the same applied voltage (absolute value).

Tudor et al. (2006) analyzed the effect of the application of an AC electric field (both constant and sweeping frequency) on the frost formation on the downward-facing surface of a horizontal surface under forced convection regime. The authors observed that when AC electric field is applied at the end of the experiment, for a short interval of time (10 s/60 s) and varying the frequency (0 Hz/7500 Hz), it is possible to obtain reductions of the frost mass accumulated up to 20% ([+ or -] 7%)/46% ([+ or -] 7%). Such elevated reductions are caused by the excitation of natural frequencies of frost crystals by the electric field, excitation that becomes particularly effective due to the variation of the electric field frequency.

Thus, the purpose of the present study is to evaluate the effectiveness of an electric field in controlling the frost formation on an upward-facing horizontal flat plate only in forced convection. The electric field intensity and uniformity, the cold surface temperature, the air velocity and the test duration were varied in order to highlight any conflicting trend and to find out the resulting optimum conditions. The air temperature and humidity are quite different from the ones adopted in the previous studies and more appropriate for heat pump applications. Moreover, different electrode geometries were used in the present study in order to experimentally compare the more studied non-uniform electric field with the less investigated uniform ones.

EXPERIMENTAL SETUP

The experimental setup used to study the influence of an electric field on the frost formation phenomenon is realized with the aim of disposing of a simple but accurate system, that allows obtaining consistent data. To increase the measurement accuracy, the experimental set-up is composed by two test sections arranged in parallel way which operate under the same thermal and fluidynamic conditions but differ only for the presence of the electric field in one of them. This arrangement is chosen because it reduces the experimental difficulties related to reproduce exactly the same test conditions during the experimental campaign and, consequently, it allows increasing experimental accuracy. Moreover, from the reference section it is possible to collect many data on frost mass accumulated without a superimposed electric field and to compare them with the data available in the open literature in order to verify that the experimental procedure is correct. A schematic of the experimental setup, with the position of measurement probes, is depicted in Figure 2.

[FIGURE 2 OMITTED]

The moist air enters the experimental apparatus passing trough a steam humidifier and a finned coil where it is first humidified and after cooled to the desired temperature and humidity conditions. Next, passing trough a variable speed centrifugal fan, air is pushed trough two parallel ducts, which are both provided with dampers and probes in order to control and measure the airflow rate. A honeycomb section connects each the air duct to the test section that is realized by a 340 mm x 210 mm x 500 mm (13.4 in x 8.3 in x 19.7 in) rectangular duct and is closed by plexiglas plates on lateral and superior faces.

The interior face of every test section is closed by an aluminium flat plate (300 mm x 200 mm x 10 mm or 11.8 in x 7.9 in x 0.4 in) that constitutes the surface where the frost forms and accumulates. Both plate are manufactured to allow the insertion, in the upper surface, of two extractable plates (300 mm x 30 mm x 2 mm or 11.8 in x 1.2 in x 0.08 in) that are weighted at the end of every experiment to evaluate the frost mass accumulated on them by subtraction of extractable plates mass (extractable plates are weighted also after drying). Both plates are cooled by a water-glycol ethylene mixture loop and the surface temperature, which is measured by twelve T thermocouples installed inside an equal number of blind holes manufactured in the lower surface, is maintained at the desired value within [+ or -] 0.1[degrees]C ([+ or -] 0.2[degrees]F) during each experiment. A cross section of the test section is depicted in Figure 3.

[FIGURE 3 OMITTED]

Finally, a data acquisition system allows monitoring and storing all the relevant test parameters (air temperature and relative humidity, air flow rate, cold plate surface temperature, high voltage applied to the electrodes). Measurement points are depicted in Figure 3, whereas measurement device characteristics are collected in Table 1.
Table 1. Measurement Device Characteristics

Measurement Object          Device               Uncertainty

Air temperature             Pt 100          [+ or -]0.5[degrees]C
                                           ([+ or -]0.9[degrees]F)

Relative humidity      Capacitive sensor      [+ or -]1% (0%...90%)

Surface temperature  Type T thermocouple    [+ or -]0.5[degrees]C
                                           ([+ or -]0.9[degrees]F)

Air velocity         Hot wire anemometer       [+ or -]0.1*v

Air flow rate          Calibrated Damper      [+ or -]5% read value

Applied voltage          High voltage       [+ or -]1% read value
                          transducer

Frost mass             Precision scale          [+ or -]0.1 g

Voltage and current  Precision multimeter  [+ or -]0.5% read value


TESTED ELECTRODES AND ELECTRIC FIELD INTENSITY CALCULATION

Four different electrodes are tested in this study. A copper flat plate (280 mm x 160 mm x 0.12 mm or 11 in x 6.3 in x 0.005 in) realizes the first electrode and it is used to create a uniform electric field. The remaining are realized by copper wire (wire diameter equal to 0.5 mm or 0.02 in), that makes the electric field non-uniform and which is arranged to obtain a straight wire (with its axis parallel to extractable plate axis), a zig-zag wire and a wire net (30 mm x 30 mm or 1.2 in x 1.2 in mesh). Every electrode is placed between two plexiglas plates (320 mm x 220 mm x 3 mm or 12.6 in x 8.7 in x 0.12 in) to avoid current leakage and high voltage sparks and its distance from the cold plate is always kept equal to 25 mm (1 in). Figure 4 shows all the tested electrodes (the electrodes are shown on a plexiglas plate).

[FIGURE 4 OMITTED]

The electric field is created grounding the flat plate and connecting a variable high voltage generator (output voltage [+ or -]0 [V.sub.DC]/[+ or -] 30000 [V.sub.DC]) to the electrode. It is worth specifying that only in one test section the electrode is effectively connected with the high voltage generator to produce the electric field, while in the remaining the electrode and the plexiglas plate are placed only to obtain the same fluidynamic conditions.

The flat plate electrode is chosen because it allows creating a uniform electric field, the simplest electric field to investigate, because, due to its simplicity, it becomes the reference electrode for the comparisons with the other ones and because there is lack of information about the effect of such an electric field in the open literature.

It is worth observing that the electric field intensity, the parameter that mainly affects the frost formation, varies with the different electrodes, although the high voltage applied to them is the same. Therefore, in order to compare the results obtained with the different electrodes, and only for this purpose, for every electrode and for every applied voltage used in the experimental campaign both the local and the mean electric field intensity are computed using a finite element software for the frostless surface. It is chosen to simulate the frostless surface because the frost thickness and the frost dielectric constant are difficult to evaluate and they are also variable as a function of time.

RESULTS

The electric field effectiveness in controlling the frost accumulation is computed measuring the frost mass accumulated on the two extractable plates and calculating the Mass Reduction Index according to the following equation:

MRI = 1 - [[m.sub.EHD]/m] (4)

The uncertainty of Mass Reduction Index is experimentally estimated and the deviation found in the experimental campaign is equal to [+ or -] 5%.

As mentioned before, the experimental campaign is carried out with the aim of investigating the influence of the electric field intensity and uniformity, of the air velocity, of the cold surface temperature and of the test duration on the frost formation. The measured parameters and their ranges (the bold values indicate the base case parameters) are shown in Table 2.
Table 2. Test Conditions

[t.sub.A], [degrees]C                    10 (50)
([degrees]F)

RH, %                                      75

[t.sub.s], [degrees]C  -11, -9, -7, -5, -3 (12.2, 15.8, 19.4, 23, 26.6)
([degrees]F)

v, m/s (ft/s)          1.5, 2, 2.5, 3, 3.5, 4 (4.9, 6.6, 8.2, 9.8, 11.5,
                                         13.1)

t, s                           3600; 7200; 10,800; 14,400

[DELTA]V, kV                 0, 3, 4, 5, 6, 7, 8, 9, 10, 11


Every test is repeated at least three times in order to improve data accuracy.

As stated before, to compare the results obtained with different electrodes, the electric field mean intensity is computed using a finite element software for the frostless surface and, therefore, the results are shown as a function of the electric field intensity.

The test conditions are chosen with the aim of disposing of conditions easily achievable, because the phenomenon studied is quite complex, but significant, especially considering a future application on a heat pump finned evaporator. This is the reason why the air temperature used is a little bit higher than temperatures used in the field of air conditioning or refrigeration, why cold surface temperatures investigated are greater than - 11[degrees]C (12.2[degrees]F) and why only forced convection is analyzed. Moreover, it is to study only DC electric field for sake of experimental simplicity.

Influence of Electric Field Intensity and Uniformity

Figure 5 shows the dependence of the Mass Reduction Index on the electric field intensity in the case of flat plate electrode (uniform electric field). The applied voltage is varied according to the values shown in Table 2 while the remaining parameters are set to the values gathered in the same table.

[FIGURE 5 OMITTED]

The experimental data show that, as a function of electric field intensity, the Mass Reduction Index increases up to a maximum and after decreases. This trend could be interpreted considering that, as stated before, the electric field either enhances or reduces the frost formation and accumulation. Consequently, with the variation of the electric field intensity one effect could prevail over the other and this explains the trend found and the presence of a maximum, before which reduction effect dominates and after which enhancement effect is more important.

The dependence of the Mass Reduction Index on the electric field intensity for all the electrodes is depicted in Figure 6. The applied voltage is varied according to the values shown in Table 2 while the remaining parameters are set to the values collected in the same table.

[FIGURE 6 OMITTED]

It is possible to state that, as a function of the electric field intensity, the Mass Reduction Index shows an increasing-decreasing trend for all the tested electrodes. Moreover, a non-uniform electric field is less effective in controlling the frost formation and accumulation, probably because, due to the electric field non-uniformity, part of the cold surface has a local electric field intensity lower than the optimum one.

Influence of Air Velocity (Reynolds Number)

Figure 7 shows the dependence of the Mass Reduction Index on the air velocity. For every electrode, the electric field intensity is set to the value that maximizes the Mass Reduction Index, the air velocity is varied according to data collected in Table 2 and the remaining parameters are set to the values shown in the same table. Reynolds number is computed using the equivalent diameter of the channel as the hydraulic diameter.

[FIGURE 7 OMITTED]

The diagram shows that the Mass Reduction Index exhibits approximately a parabolic trend as a function of the Reynolds number. A possible explanation to this trend considers that the higher is the air velocity, the higher are the heat and mass transfer coefficients and, consequently, the frost formation is promoted. Therefore, both crystal height and density increase and crystals are heavier (they break up more easily under self-weight action) and denser (they are stronger). Under the same electric field intensity and with a variation of the Reynolds number, one effect prevails over the other and this could justify the trend found. This also indicates that the electric field intensity that maximizes the Mass Reduction Index varies with air velocity.

Influence of Cold Surface Temperature

The influence of cold surface temperature on the Mass Reduction Index is shown in Figure 8. For every electrode, the electric field intensity is set to the value that maximizes the Mass Reduction Index, the cold surface temperature is varied according to data collected in Table 2 and the remaining parameters are set to the values shown in the same table.

[FIGURE 8 OMITTED]

The experimental data show that the Mass Reduction Index increases with the reduction of the cold surface temperature. This trend could be interpreted considering that the increase of temperature difference between the air stream and the cold plate promotes the frost formation and, thus, the electric field effectiveness in controlling its accumulation. Nevertheless, from the analysis of the curves related to the flat plate and the straight wire electrodes, it is possible to state that when the cold plate temperature becomes low, the Mass Reduction Index diminishes. This indicates, on one side, that the increase of frost accumulation due to a greater temperature difference prevails over the field mass reduction effect due to the electric and, on the other, that the electric field intensity that maximizes the Mass Reduction Index varies changing the cold plate temperature.

Influence of Test Duration

Figure 9 depicts the dependence of the Mass Reduction Index on test duration. For every electrode, the electric field intensity is set to the value that maximizes the Mass Reduction Index, the test duration is varied according to data gathered in Table 2 and the remaining parameters are set to the values shown in the same table.

[FIGURE 9 OMITTED]

The diagram shows that, for every duration tested, the Mass Reduction Index increases as a function of time. This indicates that the phenomenon is not in a steady-state condition and that the electric field is more and more effective in controlling the frost accumulation with the passing of time. Furthermore, the analysis of the flat plate and wire net electrode trends shows that this trend seems to achieve an asymptotic value (steady-state conditions) as observed by Joppolo et al. (2004) and Joppolo and Molinaroli (2005).

Electrical Power Required

Finally, the voltage and the current supplied to high voltage generator are measured to compute the electrical power required for maintaining electric field. The data collected are gathered in Table 3 where it is possible to note that power required is very low. Moreover, the electrical power is quite constant during the test.
Table 3. Electric Power Consumption for Maintaining Electric Field

                                High Voltage Generator Power, mW

Applied Potential,  Flat Plate  Straight Wire  Zig-Zag Wire  Wire Net
[V.sub.DC]

3000                   0.11         1.69           1.10        0.91

4000                   1.54         2.38           1.56        1.56

5000                   2.28         3.09           2.23        2.24

6000                   3.09         3.89           2.89        2.87

7000                   3.84         4.85           3.71        3.69

8000                   4.62         5.65           4.53        4.58

9000                   5.57         6.45           5.45        5.45

10000                  6.55         7.60           6.56        6.46

11000                  7.87         8.80           7.50        7.45


CONCLUSION

An experimental setup was built to study the frost formation on the simple geometry of the flat plate under the action of a DC electric field. Four different electrodes were tested and several experiments were carried out in order to understand the influence of the main parameters on the phenomenon.

The observations and the data collected could lead to the following conclusions:

1. The Mass Reduction Index reaches a maximum as a function of the electric field intensity.

2. A uniform electric field is more effective in controlling the frost accumulation.

3. Under the same electric field intensity, the Mass Reduction Index varies with the reduction of the surface temperature. The electric field intensity that maximizes the Mass Reduction Index at constant air temperature changes with the variation of the cold surface.

4. Under the same electric field intensity, the Mass Reduction Index increases as the air velocity (and correlated Reynolds number) increases. The Mass Reduction Index begins to decrease only for values of the air velocity greater than 3 m*[s.sup.-1] (9.8 ft/s)/3.5 m*[s.sup.-1] (11.5 ft/s), which are quite high for air conditioning or heat pump application.

5. The Mass Reduction Index increases with the increase of test duration, under the same electric field intensity, leading to an asymptotic value.

In summary, the study on a quite simple geometry (flat plate) but with different electrode shapes confirms interesting possibilities of frost reduction with the application of an electric field. The experiments highlighted that, due to contrasting factors, the different parameters should be optimized in order to reach a maximum in the overall performances. It is worth observing that the electrical power required to maintain the electric field is quite low if compared to the beneficial effects that it is possible to achieve such as the reduction of defrosting energy, the higher C.O.P. and the lower air side pressure drops and fan energy consumption.

ACKNOWLEDGMENTS

The authors wish to gratefully acknowledge the Carel company for the supply of the humidification section of the experimental setup.

NOMENCLATURE

E = electric field intensity, V*[m.sup.-1]

[G.sub.0] = Gibbs free energy in reference state, J

MRI = Mass Reduction Index, dimensionless

m = frost mass accumulated on test section without electric field, g

[m.sub.EHD] = frost mass accumulated on test section with electric field, g

[m.sub.F] = solidified frost mass, g

p = pressure, Pa

[p.sub.0] = reference state pressure, Pa

R = universal gas constant on mass basis, J*[g.sup.-1]*[K.sup.-1]

[r.sub.C] = critical radius of nucleation of frost crystal, m

RH = relative humidity, %

T = absolute temperature, K

[T.sub.0] = reference state absolute temperature, K

t = time, s

[t.sub.A] = air temperature, [degrees]C

[t.sub.S] = cold surface temperature, [degrees]C

v = air velocity, m*[s.sup.-1]

[gamma] = interfacial energy per unit area, J*[m.sup.-2]

[DELTA]V = applied voltage, V

[[epsilon].sub.0] = vacuum permittivity, [C.sup.2]*[N.sup.-1]*[m.sup.-2]

[[epsilon].sub.R] = relative permittivity, dimensionless

[[lambda].sub.VS] = latent heat of sublimation, J*[g.sup.-1]*[K.sup.-1]

[[rho].sub.C] = free electric charge density, C*[m.sup.-3]

[[rho].sub.I] = ice density, kg*[m.sup.-3]

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Tudor, V., Ohadi, M.M. and F.H.R. Franca. 2003. An Experimental Investigation on Frost Control Using DC and AC Electric Fields on a Horizontal Downward-Facing Plate. HVAC&R Research 9: 203-213.

Tudor, V. and M.M. Ohadi. 2006. The effect of stationary and sweeping frequency AC electric field on frost crystal removal on a cold plate. International Journal of Refrigeration 29: 669-677.

Wang, C.-C., Huang, R.-T., Sheu, W.-J. and Y.-J. Chang. 2004. Some Observation of the Frost Formation in Free Convection: With and Without the Presence of Electric Field. International Journal of Heat and Mass Transfer 47: 3491-3505.

Luca Molinaroli, PhD

Associate Member ASHRAE

C.M. Joppolo is a full professor and L. Molinaroli is an assistant professor in the Dipartimento di Energia, Politecnico di Milano, Milano, Italy.
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Author:Joppolo, Cesare Maria; Molinaroli, Luca
Publication:ASHRAE Transactions
Article Type:Report
Geographic Code:4EUIT
Date:Jul 1, 2009
Words:6004
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