It seems to me that in Mandarin, two different vowel sounds are used for words whose pinyin ends with -uan.
In some words, like 全 (quán) and 元 (yuán), the vowel sounds like "oo-en" (the nucleus/coda are "ɛn" in IPA, rhymes with English "hen").
In other words, like 換/换 (huàn) and 亂/乱 (luàn), the vowel sounds like "oo-an" (the nucleus/coda are "an" in IPA, rhymes with Mandarin "安").
Does this difference really exist, or an I imagining it? If there is a distinction, what's the rule for which pronunciation to use? It seems related to the tone and the initial consonant, but I'm not sure there is a definite pattern.