4.1. Mass Sealing Effectiveness
The mass sealing effectiveness coefficient (ξM) corresponding to the four control sections is presented in function of the AC tilt angle and of the fabric velocity for the three investigated mass flow rates.
The tilt angle configurations are presented in orange (−15°, dotted line), green (0°, dashed line) and violet (+15°, dash-dotted line), with blue pointers for an AC temperature of 15 °C and red pointers for 70°C. In
Figure 8 the control section α is considered and it is noticeable that an increase of mass flow rate from 3% to 5% corresponds to a global decrease of ξ
M for each fabric speed; a further increase of AC mass flow rate (up to 7%) reduces furtherly the ξ
M value, but a limited amount of flow leakage across the machine inlet section at the lowest fabric speed appears (negative ξ
M values). The results obtained for the section β (
Figure 9) are a consequence of this behavior and indicate that for the highest mass flow rate, with fabric moving at 20 m/min, the air curtain flow is partially directed outside of the stenter, while for all the other configurations it is moved towards into the machine.
In
Figure 10, the values of
ξM on section γ indicate that the sealing efficiency of the air curtains decreases with increasing AC mass flow rate, as the fraction of air entrained by the fabric rises. The mass sealing effectiveness coefficients computed on section δ (
Figure 11) indicate that the air curtains can seal the machine outlet only at the lowest fabric speed, when the lowest AC mass flow rate is used; for all the other configurations
ξM is negative and thus the ACs are unable to provide sealing.
As a general comment on the computational fluid dynamics results, it can be immediately observed that the efficiency and effectiveness of the air curtains mainly depend on the AC temperature and mass flow rate, while the reliance on the AC tilt angle is limited to the impact of the energy effectiveness coefficient for a few specific configurations. Detailed comments on the obtained results follow.
The results achieved for the mass sealing effectiveness coefficients show that the tilt angle of the air curtains has negligible effects on sealing, while the ratio between the mass flow rate of the air curtains and that of the air that would leak towards the ambient in absence of ACs has a key role. Air curtains with low mass flow rate are not useful for sealing the machine openings both at inlet and at outlet and they are only capable of reducing the suction effect. The sealing effect becomes significant when their mass flow rate is equal to the mass flow rate that would be ingested from the surroundings in absence of air curtains. Higher values of mass flow rate would result in a better sealing, but the sealing effectiveness would be worse as indicated by lower values of the related coefficients.
4.2. Energy Effectiveness
The energy effectiveness coefficient (ξE) is represented for the tilt angles −15°, 0° and +15° with orange (dotted), green (dashed) and violet (dash-dotted) lines respectively only for an AC temperature of 70°C as the purpose is to assess heat recovery potential.
Figure 12 shows that the use of warm air at machine inlet is beneficial for increasing the heat recovery inside of the stenter. In cases (a) and (b) with 3% and 5% of AC mass flow, respectively, the entire mass flow rate of the air curtains is ingested by the machine for each fabric speed, with positive effects on the effectiveness of heat recovery. This means that higher values of this parameter are observed for the increasing values of the fabric velocity. For the case (c) with 7% of AC mass flow rate, a positive value of
ξE is obtained only at the highest fabric speed, while for lower velocities
ξE is negative due to partial leakage of the AC mass flow rate.
In
Figure 13, it is noticeable that the tilt angle of the air curtains influences the energy effectiveness. The highest values of the
ξE are obtained with zero tilt angle air curtains. Nonetheless, that influence is reduced at higher fabric speed and higher AC mass flow rate. At the machine outlet, on section γ (
Figure 14), it is evident that the warm air of the air curtains is not correctly recovered. In fact, in most of the cases the
ξE value is close to 0 or lower when the textile velocity is medium or high, while a positive outcome is visible for low AC flow rates at low fabric velocity. The energy effectiveness coefficients at section δ in
Figure 15 confirm this trend with even lower values, thus demonstrating that the heated air is wasted at the machine outlet due to the action of the moving fabric. This clearly indicates that, for the outlet sections, air curtains at ambient temperature, or even the absence of ACs, could be preferred, as the heat recovered would be dispersed in the surrounding environment. The data state that a configuration with air curtains with tilt angles 0° or +15° and warm jets (70 °C) and a mass flow rate equal to the mass flow rate that would be ingested from the surroundings without these devices, provides the best effects. However, at the highest fabric velocity analyzed, a cold air curtain at machine outlet has better parameters. For this reason and to assess the possibility of a reduction of regenerative heat demand, even a mixed configuration with a warm inlet AC (70°C) and a cold outlet AC (15 °C) was analyzed. Since tilted air curtains would imply higher manufacturing complexities than vertical orientation, without significant improvements, only null tilt angles were considered.
In
Table 6 and
Table 7, the mass sealing effectiveness coefficient and the energy effectiveness coefficient of this configuration are enumerated and compared with those of the similar configuration with both warm air curtains. This implies that a correct selection of AC mass flow rate is fundamental to reach the optimal sealing effect, to prevent the leakage of air both for protection of the workplace environment and to avoid the waste of the warm flow of the air curtains.
4.3. Temperature Distribution
The air curtains have relevant effects on the temperature distribution inside the stenter at all the investigated fabric speeds. To assess these effects, the adiabatic wall effectiveness coefficient
ηAW was calculated:
where
is the variable temperature and
and
are the reference temperatures of the blowers (200°C) and of the air curtains (70 °C), respectively. The
ηAW coefficient is borrowed from the turbomachinery field, where it is used to evaluate the cooling effectiveness of jets in cross-flow [
34]. Here it indicates the behavior of air curtains flow and the temperature of air surrounding the fabric, considering adiabatic wall conditions. Null values of
ηAW mean that the air has the same temperature of the blowers and unitary values state that air has the same temperature of the air curtain; values ranging from 0 and 1 are related to intermediate temperatures, while values greater than 1 denote temperatures lower than that of air curtains (generated by the presence of air ingested from the ambient).
In
Figure 16,
Figure 17,
Figure 18 and
Figure 19, the
ηAW coefficient distribution of the reference configuration without air curtains (NOAC) is compared with that of the two best configurations identified, with air curtains of zero tilt angle and mass flow rate 5% in both cases and warm jets (AC0WWM) or warm and cold jets (AC0WCM). The color map is limited between 0 and 1 to clearly underline the interaction between the blowers and the air curtains, while the regions where an impact of the ambient flow is foreseen (
ηAW greater than 1) are in white. The solid line in black represents the
ηAW distribution on the fabric.
In general, an effective configuration should show a ηAW line that reaches the unitary values in correspondence of the AC positions. A ηAW line that does not reach the unitary value at the stenter outlet marks a configuration where warm air is wasted outside of the machine. As a general comment on the results, at the stenter inlet, the air curtains allow for a progressive heating of the fabric for each operating condition analyzed; the central part of the cavity is not considerably affected by air flows other than the blower jets; at the stenter outlet the behavior significantly depends on fabric speed.
Figure 16 illustrates the symmetrical distribution of
ηAW for the reference configuration without air curtains when fabric is steady. In this reference case, the negative influence of cold air entrance is clearly visible, as part of the ambient flow is mixed with the warm air from the blowers. The comparison between the reference case without air curtains at 20 m/min fabric speed and the two best configurations with ACs (
Figure 17) shows the beneficial impact of these devices. At stenter inlet, the presence of a warm air curtain determines a negligible variation of the
ηAW field, but its value on the fabric results to be lower (from 0.55 to 0.45), thus demonstrating that in this case the AC flow substitutes the ambient flow, ensuring a good sealing efficiency.
At the stenter outlet, a warm air curtain (b) seems to be preferable to a cold one (c) as the effect of suction caused by the negative pressure difference between the cavity and the environment, overrides the effect of entrainment due to fabric motion. This outcome is supported by the slowest growth of the ηAW curve on the fabric, which reaches the unitary value in correspondence of the AC slot and has lower average values, and by the fact that the injected warm air from the AC is not wasted in the ambient.
Figure 18 denotes that the use of air curtains provides some benefits for the fabric, even if its speed rises to 40 m/min. In fact, the distribution of
ηAW for the warm case shows values around the unit, also after the AC section due to the entrainment of air towards the ambient. This configuration (b) is positive for the drying process because of higher and more uniform temperatures, but the increased fabric speed makes the sealing action of the AC at the stenter outlet less effective. Instead, the use of cold air from the AC positioned near the stenter outlet (c) generates a
ηAW map like that obtained without ACs (a), thus suggesting that with this configuration there is no waste of energy and that a certain amount of sealing can be obtained also for high fabric speeds.
In
Figure 19, the worst scenario is represented. In the absence of air curtains (a), it is clearly visible that the fabric is subject to a low
ηAW value for a great portion of the machine due to a strong entrainment. Although that configuration could be of interest for the fabric warming, a significant amount of hot air exits from the outlet opening and the energy efficiency is greatly reduced. The use of a double-warm air curtain configuration (b) allows for a slight decrease of the
ηAW values near the stenter outlet (also on the fabric) but the warm flow from the AC is still wasted, as the flow is almost completely entrained towards the exit section. The configuration with the mixed warm-cold air curtains (c) reduces the portion of fabric that is subject to high temperatures, but at the same time is preferable for its higher sealing efficiency (also indicated by the mass sealing effectiveness coefficient closer to zero at control section γ shown in
Figure 14) and for its increased thermal efficiency, as there is no waste of energy towards the ambient.
A further analysis of the temperature distribution of the stenter cavity was carried out on a control rectangular area, positioned between the control sections β and γ of
Figure 7. The average temperature, the temperature ratio and the mass flow rate ratios were calculated to compare the reference configuration without air curtains and the two best configurations with air curtains (
Table 8).
By comparing the average temperatures and the temperature ratios, it is noticeable that the use of air curtains is more effective at low fabric speed and that the configuration with two warm air curtains ensures a higher temperature field as expected. The comparison of the mass flow rate ratios at control section β confirms that both AC configurations have excellent sealing effects. At control section γ it is interesting to note that this parameter quantifies the outcomes of the ηAW maps: for a fabric moving at 20 m/min and 40 m/min a warm air curtain provides better results, while at 60 m/min a cold air curtain is preferable.