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Air source heat pump water heater/chiller (ASHPWHC) units, a cooling and heating source for buildings becomes increasingly popular. However, when such a unit is operated as a heating source under low ambient temperature in winter, the formation of frost on the surface of its airside heat exchanger becomes problematic, leading to the degradation of the heat exchanger''s performance or even the shutdown of a unit. Therefore it is necessary to have a detailed investigation on the operational characteristic of the airside heat exchanger in an ASHPWHC unit under frosting in order to optimize its structural layout, thus increasing its energy efficiency and operational reliability.This paper first reports on the development of a distributed mathematical model of the airside heat exchanger under frosting in an ASHPWHC unit. The model, firstly seen in open literature, consists of a frosting sub-model and a heat exchanger sub-model. This is followed by an evaluation of operational characteristics of the airside heat exchanger under frosting in an ASHPWHC unit using the model developed. In addition, the impacts of frosting on the operational performance of an ASHPWHC unit were also evaluated. The discussions of optimizing the structural layout of finned tubes used in an airside heat exchanger and improving defrosting control measures are also included in analyzing the simulated results. [Copyright &y& Elsevier]

• The performance of MHCT heat exchangers in stagnant and flowing waters was concerned. • The effects of geometric parameters on thermal performance were described. • A mixed-convection correlation was modeled using a function of the correlations of natural and forced convections. • A dimensionless parameter dividing the convection region was proposed. For a type of multi-row helically coiled tube (MHCT) heat exchanger used in surface water heat pump systems, a series of numerical simulations for outside mixed and forced convections and experiments for natural convection are carried out with a conjugate thermal condition. The effects of geometric parameters of vertical spacing (Δ y), horizontal spacing (Δ x), tube length (L), and number of rows (K) on the outside Nusselt number (Nu o) are detailed for 11 heat exchangers in different configurations. The results show that an increase in Δ y or a decrease in K can increase Nu o both in the forced and natural convections, whereas, there is a slight change in Nu o as K > 2 or Δ y − d o ≥ d o. Furthermore, Nu o is insensitive to changes in Δ x and L. The correlations for natural and forced convections are first developed by considering the effects of these geometric parameters. Then, the correlation for mixed convection is modeled using a function of the natural and forced convections. A dimensionless parameter, G r / R e 1 / r , is proposed to divide the convection region and can be used to select the corresponding correlation for the design of the MHCT heat exchanger in different surface-water bodies. [ABSTRACT FROM AUTHOR]

• A detailed icing model is established considering convection variation along the tube length. • The effects of operating and geometric parameters on icing are discussed. • The variations on thickness and distribution length of ice are analyzed. • The reduction percentage in heat transfer rate is proportional to the increase of average ice thickness. • Reference charts are drawn to determine the minimum vertical spacing to avoid ice filling the gap. Helical coil, especially a multirow helically coiled tube configuration, is a particular type of heat exchanger commonly used in surface water heat pump systems. When the fluid temperature decreases, the icing on the outer wall influences the thermal behavior of the heat exchanger. In this study, a detailed heat transfer model was established for icing and non-icing conditions. The correlations of outside local convection along the tube length were modeled for flowing and stagnant water bodies, considering the flow direction and the effects of geometric parameters. Simulations of icing and non-icing conditions were conducted to analyze the influence of icing. The results show that increasing the medium velocity, inlet temperature of the medium, and surface-water temperature or decreasing the surface-water velocity could significantly increase the average thickness and the distribution length of the ice. As regards geometric parameters, ice will be produced more in a smaller vertical spacing or a higher number of rows, whereas it is insensitive to changes in horizontal spacing and tube length. The reduced percentage of the heat exchange rate is 0.62%/mm for an increase in average ice thickness; however, it could be greater when ice fills the gap. Reference charts are finally provided for vertical spacing to avoid ice filling the gap to be used in the design phase. [ABSTRACT FROM AUTHOR]

• SiO 2 particles extended the induction period of the combined fouling. • SiO 2 particles increased the asymptotic fouling resistance of combined fouling. • For combined fouling, plain tube had a longer induction period than enhanced tubes. • The variation profile of the accelerated combined fouling after scaling down is similar to that in a real system. This study investigated the interaction between particulate fouling and precipitation fouling on two different enhanced tubes and a plain tube in a shell-and-tube condenser. Three kinds of fouling, including particulate fouling, precipitation fouling, and combined fouling, were measured continuously for more than 720 h in three individual tests, respectively. For the particulate fouling, three tubes suffered a similar fouling process. The asymptotic fouling resistance of the precipitation fouling on enhanced tubes was lower than on the plain tube. However, for the combined fouling, the enhanced tube with a larger start number experienced the greatest fouling. On the same tube, the asymptotic fouling resistances ranked from least to greatest in the following order: particulate fouling, precipitation fouling, combined fouling; and the durations of the induction period ranked as: particulate fouling, precipitation fouling, combined fouling. As for the combined fouling existent in real cooling water systems, the fouling induction period on the plain tube was longer than that of enhanced tubes. Compared with the particulate fouling (SiO 2) and precipitation fouling (CaCO 3), which had a high bond strength on the tube surface, the combined fouling was soft and easily removed by flow erosion or collision with big particles during the induction period. This indicated that particles mixed in the water could extend the induction period of combined fouling. However, once the combined fouling started to grow, these particles sticking on the heat transfer surface provided nucleation sites for the crystallization process and, thus, increased the fouling thermal resistance significantly. In addition, the results indicated that the variation profile of the accelerated combined fouling after scaling down is similar to the fouling process in a real system. [ABSTRACT FROM AUTHOR]

Highlights • A quantitative study of the frost distribution and growth on FTHX was reported. • Frost pattern between windward and leeward sides of FTHX was compared. • Frost thickness, density and mass on different parts of FTHX were analyzed. • The effects of frosting conditions on frost distribution and growth were studied. Abstract Recently, the authors have published a paper which provided the frost distribution and growth along the airflow direction of finned tube heat exchangers (FTHXs), and demonstrated that some frost was accumulated on the edge of windward fins at a given frosting condition. As a follow-up to the work reported in the previous paper, in the current paper, the effects of frosting conditions on frost distribution and growth characteristics of FTHXs with different fin pitches were experimentally investigated. The results showed that the protruded thickness of frost on the edge of windward fins and the ratio of the frost mass on the edge of windward fins to that on the entire FTHX were both increased with the increases in air temperature (T a) and relative humidity (RH a), but decreased with an increase in initial air face velocity (v a). In addition, the effects of RH a on the mass ratio were more significant than those of T a and v a. Furthermore, the averaged frost density on the edge of windward fins was increased with an increase in RH a ; and that on the surfaces of fins and tubes was increased with an increase in v a , but decreased with an increase in RH a. [ABSTRACT FROM AUTHOR]

Fouling deposit is a common issue on the heat transfer surface caused by the impurity of working water. Many researchers conducted experimental test to study the relationship between the fouling thermal resistance and operating parameters, such as water quality, tube geometry, and liquid velocity, targeting at developing the accurate correlation of fouling thermal resistance on heat transfer tubes. The accurate test of fouling thermal resistance is critical for investigators. In fouling test, with the fouling deposit on the internal surface, both the liquid (water) velocity through the tube and the heat flux of the test tube deviated automatically. Although testers usually tried to adjust the water velocity and heat flux back to the original point, it is hard to be realized, thus the water velocity and heat flux deviated somehow inevitably. In fact, the variations of water velocity and heat flux would cause the change of overall thermal resistance of test tubes, which should be separated from the change caused by fouling deposit. This process could be named as “decoupling”. This paper analyzed the effect of deviations of water velocity and heat flux on the test results of fouling resistance quantitatively based on experimental test, and a decoupling method and formulas were developed. One set of accelerated fouling test was conducted and result shows the fouling resistance with decoupling and non-decoupling had a maximum difference of 0.000002124 m 2  K/W for tube 1, and 0.000002363 m 2  K/W for Tube 2, 0.000001316 m 2  K/W for tube 3. [ABSTRACT FROM AUTHOR]

• SiO 2 particles increased the sticking probability in the combined fouling. • Enhanced tubes had lower sticking probability but higher deposit bond strength than plain tube. • The synergistic of the flow field and the temperature field affected fouling deposit. Sticking probability (P) and deposit bond strength (ξ) are the two most important factors that determine the fouling process, but no calculation correlations were found for P and ξ in current models. This study investigated the interaction between particulate fouling and precipitation fouling based on the values of P and ξ , taking into account the growth characteristics of the combination of particulate fouling and precipitation fouling on the micro-surface of the heat transfer tube. Three kinds of fouling were measured continuously over 720 h in each of three individual tests: particulate fouling, precipitation fouling, and the combined fouling. In order to recognize the growth mechanism of the combined fouling on the heat transfer surface and to solve the engineering problems, this study provides a reliable method to calculated P and ξ. The results indicated that the solution of CaCO 3 mixed with the particles of SiO 2 at the mass ratio of 1:1 (CaCO 3 :SiO 2) improved sticking probability. And the enhanced tubes, the helical ridge tube with start-number of 45 and start-number of 10, compared with the plain tube, would not easily stick, but hardly denude by flow which reduced the sticking probability and increased the deposit bond strength. For the helical ridge tube with start-number of 45, when compared with the helical ridge tube with start-number of 10, the particles were more likely to stick to the surface of, but the precipitation fouling was harder to occur. It was observed that deposits were difficult to remove once they were deposited by the particulate fouling or precipitation fouling on the surface of the helical ridge tube with start-number of 45. [ABSTRACT FROM AUTHOR]