1、2009 ASHRAE 737ABSTRACTThe paper discusses the results of an experimental study carried out to investigate the influence of a DC electric field on frost formation and accumulation on an upward-facing flat plate. An experimental setup was built to investigate the influ-ence of the electric field inte
2、nsity and uniformity on frost mass accumulated on the flat surface and to analyze its dependence on cold surface temperature, air velocity and test duration. In the experiments different electrode geometries were adopted allowing to compare uniform electric field with non-uniform ones. Every experim
3、ent was conducted under controlled envi-ronmental conditions (air dry bulb temperature and air rela-tive humidity), which were appropriate for heat pump applications and not well investigated in open literature. The results show that the electric field allows reducing the frost mass accumulated on f
4、lat plate surface up to a value of 26% while the power required to maintain the electric field is quite low. They also suggest that a uniform electric field could achieve better results even if its applicability has to be deeply considered.INTRODUCTIONFrost formation and accumulation on the external
5、 surface of an evaporator is a very common issue because the thermo-dynamic conditions at which the heat exchanger works are often characterized by temperature (refrigerant side), temper-ature and humidity (moist air side) such that frost formation and accumulation can occur (namely, evaporation tem
6、perature lower than 0C 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 therm
7、al 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 remove
8、d from evaporative coil by defrosting cycles that, however, involves the service inter-ruption 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 p
9、ressure 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 FIELDFrost for
10、mation 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 fie
11、ld 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 follow-ing expressi
12、ons which relate the saturation pressure of the water vapor and the frost crystal critical radius of nucleation with the electric field intensity:Frost Accumulation Control on an Upward-Facing Horizontal Flat Plate Using Electric FieldCesare Maria Joppolo Luca Molinaroli, PhDAssociate Member ASHRAEC
13、.M. Joppolo is a full professor and L. Molinaroli is an assistant professor in the Dipartimento di Energia, Politecnico di Milano, Milano, Italy. LO-09-069 2009, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions 2009, v
14、ol. 115, part 2. For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAEs prior written permission.738 ASHRAE Transactions(1)(2)The two equations above show that the action of the elec-tric field causes, at the sam
15、e temperature, the reduction of water vapor saturation pressure (1) and the decrease of the crit-ical 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 sat
16、urated 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 t
17、he second parameter (crystal height to diam-eter ratio) causes an increase of the first (electric field inten-sity) 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
18、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 pref-erential direction, the d
19、irection 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 nearl
20、y 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)
21、. Figure 1 (from Libbrecht 1999) illustrates the frost crystal morphology in absence and presence of the elec-tric field showing that the crystals growing under the action of the electric field are needled-shaped, longer and thinner crys-tals when compared to the analogous ones solidified without a
22、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:(3)It is possible to observe that, as a function of the electric fie
23、ld intensity, there is a conflicting trend due to the fact that with the increase of the electric field intensity, both enhance-ment 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 theor
24、etical 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 trans
25、fer mechanisms which reduce up to annul the frost reduction mechanism.LITERATURE REVIEWExperimental 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 cyl
26、indrical pipe).Schaefer (1950) obtained the first qualitative observa-tions 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 struc-ture in which they solidify is n
27、eedled-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 e
28、lectric 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 m/s (0.003 in/s) (between the ten and one hundred times greater than the normal growth veloc
29、ity) and the shape that crystals assume is sharp, thin and branchless. The authors observed moreover the spontaneous break of the crystals and the migra-tion of fragments towards the electrode opposite to the cold surface as an indication of the presence of an electric charge on the crystals. Such p
30、henomenon appeared emphasized by the pp0- expVSR-1T0-1T-1RT-dG0dmF-0R1()E22I-+=rC2IRTpp0-VSR- 1TT0-0R1()E22I-+ln-=fE=CE12-E212- E2-T+Figure 1 Frost crystals formed without an electric field (I) and under an electric field (II). Image III shows the change of the morphology of a frost crystal that sol
31、idifies at first without and subsequently under the action of an electric field (Libbrecht 1999).ASHRAE Transactions 739sudden reversal of the electric field polarity while it was inde-pendent from the polarity itself or the frequency of the field.Maybank et al. (1967) analyzed the influence of an e
32、lec-tric field on the frost crystal growth. They found that, for elec-tric field intensity greater than 20 kV/m (6.1 kV/ft), the crystal growth velocity increases while the crystals are more numer-ous and they solidify in thinner and more fragile structures compared to the crystals formed without a
33、superimposed elec-tric 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
34、 mass in correspondence of a corona current equal to 200 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
35、 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 coeff
36、icient 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
37、 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 tempera-ture 17C (63F), humidit
38、y ratio 0.0085 kg/kgDA, 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 red
39、uction of the frost mass equal to 20% when the corona currents intensity is lower than 5 A and an increase of the frost mass of the order of 100% when the corona currents intensity is approximately equal to 120 A.Bloshteyn et al. (1999) studied the influence of an electric field on the frost formati
40、on 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 a
41、n 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 ove
42、r a flat plate arranged both vertically and horizontally. The flow regime was natural convection in the first case and both natural and forced convec-tion 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 3
43、0.8% 5% (applied voltage 35 kV) or equal to 34.7% 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 (pa
44、rallel 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%
45、 (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
46、al. (2003) investigated the influence of the elec-tric 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 convec-tion regime for AC electric fields. In both cases, the electric
47、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 equa
48、l 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 dura-tion. The forced convection results show the same results.Wang et al. (2004) studied the influence of t
49、he 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 tempera-ture of the heat exchanger (increase equal to
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